diff --git a/packages/ipfs-message-port-client/src/client/query.js b/packages/ipfs-message-port-client/src/client/query.js
index 749683f6ed..8f4369bb74 100644
--- a/packages/ipfs-message-port-client/src/client/query.js
+++ b/packages/ipfs-message-port-client/src/client/query.js
@@ -2,7 +2,7 @@
  * @typedef {Object} QueryOptions
  * @property {AbortSignal} [signal]
  * @property {number} [timeout]
- * @property {Transferable[]} [transfer]
+ * @property {Set<Transferable>} [transfer]
  */
 
 /**
@@ -49,7 +49,7 @@ export class Query {
   /**
    * Data that will be transferred over message channel.
    *
-   * @returns {Transferable[]|void}
+   * @returns {Set<Transferable>|void}
    */
   transfer () {
     return this.input.transfer
diff --git a/packages/ipfs-message-port-client/src/client/transport.js b/packages/ipfs-message-port-client/src/client/transport.js
index d9605e1691..b717cd6f9d 100644
--- a/packages/ipfs-message-port-client/src/client/transport.js
+++ b/packages/ipfs-message-port-client/src/client/transport.js
@@ -180,8 +180,8 @@ export class MessageTransport {
         id,
         input: query.toJSON()
       },
-      // @ts-ignore - TS seems to want second arg to postMessage to not be undefined
-      [...new Set(query.transfer() || [])]
+      // @ts-expect-error - Type signature does not expect 2nd undefined arg
+      query.transfer()
     )
   }
 
diff --git a/packages/ipfs-message-port-client/src/core.js b/packages/ipfs-message-port-client/src/core.js
index 9e0ea86e1b..d49ae0e3c7 100644
--- a/packages/ipfs-message-port-client/src/core.js
+++ b/packages/ipfs-message-port-client/src/core.js
@@ -71,7 +71,7 @@ export class CoreClient extends Client {
  */
 CoreClient.prototype.addAll = async function * addAll (input, options = {}) {
   const { timeout, signal } = options
-  const transfer = [...(options.transfer || [])]
+  const transfer = options.transfer || new Set()
   const progressCallback = options.progress
     ? encodeCallback(options.progress, transfer)
     : undefined
@@ -99,7 +99,7 @@ CoreClient.prototype.addAll = async function * addAll (input, options = {}) {
  */
 CoreClient.prototype.add = async function add (input, options = {}) {
   const { timeout, signal } = options
-  const transfer = [...(options.transfer || [])]
+  const transfer = options.transfer || new Set()
   const progressCallback = options.progress
     ? encodeCallback(options.progress, transfer)
     : undefined
@@ -184,7 +184,7 @@ const identity = (v) => v
  * given input.
  *
  * @param {ImportCandidate} input
- * @param {Transferable[]} transfer
+ * @param {Set<Transferable>} transfer
  * @returns {Promise<EncodedAddInput>}
  */
 const encodeAddInput = async (input, transfer) => {
@@ -242,7 +242,7 @@ const encodeAddInput = async (input, transfer) => {
  * given input.
  *
  * @param {ImportCandidateStream} input
- * @param {Transferable[]} transfer
+ * @param {Set<Transferable>} transfer
  * @returns {EncodedAddAllInput}
  */
 const encodeAddAllInput = (input, transfer) => {
@@ -279,7 +279,7 @@ const encodeAddAllInput = (input, transfer) => {
  * effective strategy.
  *
  * @param {ImportCandidate} content
- * @param {Transferable[]} transfer
+ * @param {Set<Transferable>} transfer
  * @returns {EncodedAddInput}
  */
 const encodeAsyncIterableContent = (content, transfer) => {
@@ -303,7 +303,7 @@ const encodeAsyncIterableContent = (content, transfer) => {
 
 /**
  * @param {ImportCandidate} content
- * @param {Transferable[]} transfer
+ * @param {Set<Transferable>} transfer
  * @returns {EncodedAddInput}
  */
 const encodeIterableContent = (content, transfer) => {
@@ -329,7 +329,7 @@ const encodeIterableContent = (content, transfer) => {
 
 /**
  * @param {ToFile | ToDirectory} file
- * @param {Transferable[]} transfer
+ * @param {Set<Transferable>} transfer
  * @returns {EncodedFileInput | EncodedDirectoryInput}
  */
 const encodeFileObject = ({ path, mode, mtime, content }, transfer) => {
@@ -349,7 +349,7 @@ const encodeFileObject = ({ path, mode, mtime, content }, transfer) => {
 
 /**
  * @param {ToContent|undefined} content
- * @param {Transferable[]} transfer
+ * @param {Set<Transferable>} transfer
  * @returns {EncodedFileContent}
  */
 const encodeFileContent = (content, transfer) => {
diff --git a/packages/ipfs-message-port-client/src/dag.js b/packages/ipfs-message-port-client/src/dag.js
index 6875f9e9ce..8dcf8e0b8d 100644
--- a/packages/ipfs-message-port-client/src/dag.js
+++ b/packages/ipfs-message-port-client/src/dag.js
@@ -62,7 +62,7 @@ DAGClient.prototype.resolve = async function resolve (cid, options = {}) {
 
 /**
  * @param {string|CID} input
- * @param {Transferable[]} [transfer]
+ * @param {Set<Transferable>} [transfer]
  * @returns {string|EncodedCID}
  */
 const encodeCIDOrPath = (input, transfer) => {
diff --git a/packages/ipfs-message-port-client/src/interface.ts b/packages/ipfs-message-port-client/src/interface.ts
index 693f9172e6..8f5ed7639b 100644
--- a/packages/ipfs-message-port-client/src/interface.ts
+++ b/packages/ipfs-message-port-client/src/interface.ts
@@ -2,5 +2,5 @@
 // JSDoc syntax or that result in a different behaviour when typed in JSDoc.
 
 export interface MessagePortClientOptions {
-  transfer?: Transferable[]
+  transfer?: Set<Transferable>
 }
diff --git a/packages/ipfs-message-port-protocol/README.md b/packages/ipfs-message-port-protocol/README.md
index 04a8eede33..da4a458c58 100644
--- a/packages/ipfs-message-port-protocol/README.md
+++ b/packages/ipfs-message-port-protocol/README.md
@@ -146,7 +146,7 @@ port2.onmessage = async ({data}) => {
 
 ### Callback
 
-Primitive callbacks that take single parameter supported by [structured cloning algorithm][] like progress callback used across IPFS APIs can be encoded / decoded. Unilke most encoders `transfer` argument is required (because value is encoded to a [MessagePort][] that can only be transferred)
+Primitive callbacks that take single parameter supported by [structured cloning algorithm][] like progress callback used across IPFS APIs can be encoded / decoded. Unlike most encoders `transfer` argument is required (because value is encoded to a [MessagePort][] that can only be transferred)
 
 ```js
 import { encodeCallback, decodeCallback } from 'ipfs-message-port-protocol/core'
@@ -186,4 +186,3 @@ Check out our [contributing document](https://github.com/ipfs/community/blob/mas
 ## License
 
 [![FOSSA Status](https://app.fossa.io/api/projects/git%2Bgithub.com%2Fipfs%2Fjs-ipfs.svg?type=large)](https://app.fossa.io/projects/git%2Bgithub.com%2Fipfs%2Fjs-ipfs?ref=badge_large)
-
diff --git a/packages/ipfs-message-port-protocol/src/block.js b/packages/ipfs-message-port-protocol/src/block.js
index 18cc0c5a86..f4ee247cb4 100644
--- a/packages/ipfs-message-port-protocol/src/block.js
+++ b/packages/ipfs-message-port-protocol/src/block.js
@@ -15,11 +15,11 @@
  * copy.
  *
  * @param {Uint8Array} data
- * @param {Transferable[]} [transfer]
+ * @param {Set<Transferable>} [transfer]
  */
 export const encodeBlock = (data, transfer) => {
   if (transfer) {
-    transfer.push(data.buffer)
+    transfer.add(data.buffer)
   }
   return data
 }
diff --git a/packages/ipfs-message-port-protocol/src/cid.js b/packages/ipfs-message-port-protocol/src/cid.js
index 2934bf2c2b..a13cc23329 100644
--- a/packages/ipfs-message-port-protocol/src/cid.js
+++ b/packages/ipfs-message-port-protocol/src/cid.js
@@ -14,12 +14,12 @@ import { CID } from 'multiformats/cid'
  * will be added for the transfer list.
  *
  * @param {CID} cid
- * @param {Transferable[]} [transfer]
+ * @param {Set<Transferable>} [transfer]
  * @returns {EncodedCID}
  */
 export const encodeCID = (cid, transfer) => {
   if (transfer) {
-    transfer.push(cid.multihash.bytes.buffer)
+    transfer.add(cid.multihash.bytes.buffer)
   }
   return cid
 }
diff --git a/packages/ipfs-message-port-protocol/src/core.js b/packages/ipfs-message-port-protocol/src/core.js
index 6dfd0bad1a..6d536b7b5e 100644
--- a/packages/ipfs-message-port-protocol/src/core.js
+++ b/packages/ipfs-message-port-protocol/src/core.js
@@ -92,16 +92,19 @@ export const decodeIterable = async function * ({ port }, decode) {
 /**
  * @template I,O
  * @param {AsyncIterable<I>|Iterable<I>} iterable
- * @param {function(I, Transferable[]):O} encode
- * @param {Transferable[]} transfer
+ * @param {function(I, Set<Transferable>):O} encode
+ * @param {Set<Transferable>} transfer
  * @returns {RemoteIterable<O>}
  */
 export const encodeIterable = (iterable, encode, transfer) => {
   const { port1: port, port2: remote } = new MessageChannel()
-  /** @type {Transferable[]} */
-  const itemTransfer = []
   /** @type {Iterator<I>|AsyncIterator<I>} */
   const iterator = toIterator(iterable)
+  // Note that port.onmessage will receive multiple 'next' method messages.
+  // Instead of allocating set every time we allocate one here and recycle
+  // it on each 'next' message.
+  /** @type {Set<Transferable>} */
+  const itemTransfer = new Set()
 
   port.onmessage = async ({ data: { method } }) => {
     switch (method) {
@@ -112,12 +115,15 @@ export const encodeIterable = (iterable, encode, transfer) => {
             port.postMessage({ type: 'next', done: true })
             port.close()
           } else {
-            itemTransfer.length = 0
-            port.postMessage(
+            itemTransfer.clear()
+            const encodedValue = encode(value, itemTransfer)
+
+            postMessage(
+              port,
               {
                 type: 'next',
                 done: false,
-                value: encode(value, itemTransfer)
+                value: encodedValue
               },
               itemTransfer
             )
@@ -144,7 +150,7 @@ export const encodeIterable = (iterable, encode, transfer) => {
     }
   }
   port.start()
-  transfer.push(remote)
+  transfer.add(remote)
 
   return { type: 'RemoteIterable', port: remote }
 }
@@ -170,31 +176,41 @@ const toIterator = iterable => {
 
 /**
  * @param {Function} callback
- * @param {Transferable[]} transfer
+ * @param {Set<Transferable>} transfer
  * @returns {RemoteCallback}
  */
 export const encodeCallback = (callback, transfer) => {
   // eslint-disable-next-line no-undef
   const { port1: port, port2: remote } = new MessageChannel()
   port.onmessage = ({ data }) => callback.apply(null, data)
-  transfer.push(remote)
+  transfer.add(remote)
   return { type: 'RemoteCallback', port: remote }
 }
 
 /**
  * @template T
  * @param {RemoteCallback} remote
- * @returns {function(T[]):void | function(T[], Transferable[]):void}
+ * @returns {function(T[]):void | function(T[], Set<Transferable>):void}
  */
 export const decodeCallback = ({ port }) => {
   /**
    * @param {T[]} args
-   * @param {Transferable[]} [transfer]
+   * @param {Set<Transferable>} [transfer]
    * @returns {void}
    */
-  const callback = (args, transfer = []) => {
-    port.postMessage(args, [...new Set(transfer)])
+  const callback = (args, transfer) => {
+    postMessage(port, args, transfer)
   }
 
   return callback
 }
+
+/**
+ * @param {MessagePort} port
+ * @param {any} message
+ * @param {Iterable<Transferable>} [transfer]
+ */
+const postMessage = (port, message, transfer) =>
+  // @ts-expect-error - Built in types expect Transferable[] but it really
+  // should be Iterable<Transferable>
+  port.postMessage(message, transfer)
diff --git a/packages/ipfs-message-port-protocol/src/dag.js b/packages/ipfs-message-port-protocol/src/dag.js
index 540a3f326d..c375796edc 100644
--- a/packages/ipfs-message-port-protocol/src/dag.js
+++ b/packages/ipfs-message-port-protocol/src/dag.js
@@ -41,7 +41,7 @@ export const decodeNode = ({ dagNode, cids }) => {
  * this node will be added to transfer so they are moved across without copy.
  *
  * @param {DAGNode} dagNode
- * @param {Transferable[]} [transfer]
+ * @param {Set<Transferable>} [transfer]
  * @returns {EncodedDAGNode}
  */
 export const encodeNode = (dagNode, transfer) => {
@@ -58,7 +58,7 @@ export const encodeNode = (dagNode, transfer) => {
  *
  * @param {DAGNode} value
  * @param {CID[]} cids
- * @param {Transferable[]} [transfer]
+ * @param {Set<Transferable>} [transfer]
  * @returns {void}
  */
 const collectNode = (value, cids, transfer) => {
@@ -70,11 +70,11 @@ const collectNode = (value, cids, transfer) => {
       encodeCID(cid, transfer)
     } else if (value instanceof ArrayBuffer) {
       if (transfer) {
-        transfer.push(value)
+        transfer.add(value)
       }
     } else if (ArrayBuffer.isView(value)) {
       if (transfer) {
-        transfer.push(value.buffer)
+        transfer.add(value.buffer)
       }
     } else if (Array.isArray(value)) {
       for (const member of value) {
diff --git a/packages/ipfs-message-port-protocol/src/rpc.ts b/packages/ipfs-message-port-protocol/src/rpc.ts
index 0e2393da78..e340097e99 100644
--- a/packages/ipfs-message-port-protocol/src/rpc.ts
+++ b/packages/ipfs-message-port-protocol/src/rpc.ts
@@ -6,18 +6,18 @@ export type Remote<T extends Record<string, unknown>> = {
   [K in keyof T]: Procedure<T[K]>
 }
 
-type Return<T> = T extends Promise<infer U>
+export type Return<T> = T extends Promise<infer U>
   ? Promise<U & TransferOptions>
   : Promise<T & TransferOptions>
 
 export interface QueryOptions {
   signal?: AbortSignal
   timeout?: number
-  transfer?: Transferable[]
+  transfer?: Set<Transferable>
 }
 
 export interface TransferOptions {
-  transfer?: Transferable[]
+  transfer?: Set<Transferable>
 }
 
 export type NonUndefined<A> = A extends undefined ? never : A
diff --git a/packages/ipfs-message-port-protocol/test/block.browser.js b/packages/ipfs-message-port-protocol/test/block.browser.js
index 51e7144734..b7477d40bf 100644
--- a/packages/ipfs-message-port-protocol/test/block.browser.js
+++ b/packages/ipfs-message-port-protocol/test/block.browser.js
@@ -22,7 +22,7 @@ describe('block (browser)', function () {
     it('should decode Block over message channel & transfer bytes', async () => {
       const blockIn = uint8ArrayFromString('hello')
 
-      const transfer = []
+      const transfer = new Set()
 
       const blockOut = await move(encodeBlock(blockIn, transfer), transfer)
 
diff --git a/packages/ipfs-message-port-protocol/test/cid.browser.js b/packages/ipfs-message-port-protocol/test/cid.browser.js
index ce0f4e460c..95e69b096a 100644
--- a/packages/ipfs-message-port-protocol/test/cid.browser.js
+++ b/packages/ipfs-message-port-protocol/test/cid.browser.js
@@ -26,7 +26,7 @@ describe('cid (browser)', function () {
 
     it('should decode CID and transfer bytes', async () => {
       const cidIn = CID.parse('Qmd7xRhW5f29QuBFtqu3oSD27iVy35NRB91XFjmKFhtgMr')
-      const transfer = []
+      const transfer = new Set()
       const cidDataIn = encodeCID(cidIn, transfer)
       const cidDataOut = await move(cidDataIn, transfer)
       const cidOut = decodeCID(cidDataOut)
diff --git a/packages/ipfs-message-port-protocol/test/core.browser.js b/packages/ipfs-message-port-protocol/test/core.browser.js
index 3315af463c..8eba3c61c1 100644
--- a/packages/ipfs-message-port-protocol/test/core.browser.js
+++ b/packages/ipfs-message-port-protocol/test/core.browser.js
@@ -26,7 +26,7 @@ describe('core', function () {
           deliver = resolve
         })
 
-      const transfer = []
+      const transfer = new Set()
       const remote = decodeCallback(
         await move(encodeCallback(callback, transfer), transfer)
       )
@@ -49,7 +49,7 @@ describe('core', function () {
           deliver = resolve
         })
 
-      const transfer = []
+      const transfer = new Set()
       const remote = decodeCallback(
         await move(encodeCallback(callback, transfer), transfer)
       )
@@ -74,7 +74,7 @@ describe('core', function () {
         yield { items: [uint8ArrayFromString('bla'), uint8ArrayFromString('bla')] }
       }
 
-      const transfer = []
+      const transfer = new Set()
 
       const remote = decodeIterable(
         await move(
@@ -118,7 +118,7 @@ describe('core', function () {
         }
       }
 
-      const transfer = []
+      const transfer = new Set()
 
       const remote = decodeIterable(
         await move(
@@ -159,7 +159,7 @@ describe('core', function () {
         throw Error('Producer Boom!')
       }
 
-      const transfer = []
+      const transfer = new Set()
 
       const remote = decodeIterable(
         await move(
@@ -200,7 +200,7 @@ describe('core', function () {
         }
       }
 
-      const transfer = []
+      const transfer = new Set()
 
       const remote = decodeIterable(
         await move(
@@ -245,14 +245,14 @@ describe('core', function () {
         yield * outgoing
       }
 
-      const transfer = []
+      const transfer = new Set()
 
       const remote = decodeIterable(
         await move(
           encodeIterable(
             iterate(),
             (data, transfer) => {
-              transfer.push(data.buffer)
+              transfer.add(data.buffer)
               return data
             },
             transfer
@@ -287,7 +287,7 @@ describe('core', function () {
         yield { items: [uint8ArrayFromString('bla'), uint8ArrayFromString('bla')] }
       }
 
-      const transfer = []
+      const transfer = new Set()
 
       const remote = decodeIterable(
         await move(
@@ -331,7 +331,7 @@ describe('core', function () {
         }
       }
 
-      const transfer = []
+      const transfer = new Set()
 
       const remote = decodeIterable(
         await move(
@@ -371,7 +371,7 @@ describe('core', function () {
         throw Error('Producer Boom!')
       }
 
-      const transfer = []
+      const transfer = new Set()
 
       const remote = decodeIterable(
         await move(
@@ -411,7 +411,7 @@ describe('core', function () {
         }
       }
 
-      const transfer = []
+      const transfer = new Set()
 
       const remote = decodeIterable(
         await move(
@@ -455,14 +455,14 @@ describe('core', function () {
         yield * outgoing
       }
 
-      const transfer = []
+      const transfer = new Set()
 
       const remote = decodeIterable(
         await move(
           encodeIterable(
             iterate(),
             (data, transfer) => {
-              transfer.push(data.buffer)
+              transfer.add(data.buffer)
               return data
             },
             transfer
diff --git a/packages/ipfs-message-port-protocol/test/dag.browser.js b/packages/ipfs-message-port-protocol/test/dag.browser.js
index 8a392a2b5c..1b78c3d92c 100644
--- a/packages/ipfs-message-port-protocol/test/dag.browser.js
+++ b/packages/ipfs-message-port-protocol/test/dag.browser.js
@@ -58,7 +58,7 @@ describe('dag (browser)', function () {
           link: CID.parse(cid2)
         }
       }
-      const transfer = []
+      const transfer = new Set()
 
       const nodeOut = decodeNode(
         await move(encodeNode(nodeIn, transfer), transfer)
@@ -80,7 +80,7 @@ describe('dag (browser)', function () {
         }
       })
 
-      expect(transfer).to.containSubset(
+      expect([...transfer]).to.containSubset(
         [{ byteLength: 0 }, { byteLength: 0 }, { byteLength: 0 }],
         'transferred buffers were cleared'
       )
diff --git a/packages/ipfs-message-port-protocol/test/dag.spec.js b/packages/ipfs-message-port-protocol/test/dag.spec.js
index 8e80819438..110a6e841e 100644
--- a/packages/ipfs-message-port-protocol/test/dag.spec.js
+++ b/packages/ipfs-message-port-protocol/test/dag.spec.js
@@ -10,7 +10,7 @@ describe('dag', function () {
   this.timeout(10 * 1000)
 
   describe('encodeNode / decodeNode', () => {
-    it('shoud encode node', () => {
+    it('should encode node', () => {
       const cid1 = CID.parse(
         'bafyreic6f672hnponukaacmk2mmt7vs324zkagvu4hcww6yba6kby25zce'
       )
@@ -34,7 +34,7 @@ describe('dag', function () {
       expect(data.cids).to.include(cid2)
     })
 
-    it('shoud encode and add buffers to transfer list', () => {
+    it('should encode and add buffers to transfer list', () => {
       const cid1 = CID.parse(
         'bafyreic6f672hnponukaacmk2mmt7vs324zkagvu4hcww6yba6kby25zce'
       )
@@ -55,7 +55,7 @@ describe('dag', function () {
         }
       }
 
-      const transfer = []
+      const transfer = new Set()
       const data = encodeNode(dagNode, transfer)
 
       expect(data.dagNode).to.be.equal(dagNode)
@@ -64,14 +64,14 @@ describe('dag', function () {
       expect(data.cids).to.include(cid1)
       expect(data.cids).to.include(cid2)
 
-      expect(transfer).to.be.an.instanceOf(Array)
-      expect(transfer).to.have.property('length', 3)
+      expect(transfer).to.be.an.instanceOf(Set)
+      expect(transfer).to.have.property('size', 3)
       expect(transfer).to.include(cid1.multihash.bytes.buffer)
       expect(transfer).to.include(cid2.multihash.bytes.buffer)
       expect(transfer).to.include(hi.buffer)
     })
 
-    it('shoud decode node', () => {
+    it('should decode node', () => {
       const cid1 = CID.parse(
         'bafyreic6f672hnponukaacmk2mmt7vs324zkagvu4hcww6yba6kby25zce'
       )
@@ -92,7 +92,7 @@ describe('dag', function () {
         }
       }
 
-      const transfer = []
+      const transfer = new Set()
       const data = encodeNode(dagNode, transfer)
 
       expect(data.dagNode).to.be.equal(dagNode)
@@ -101,8 +101,8 @@ describe('dag', function () {
       expect(data.cids).to.include(cid1)
       expect(data.cids).to.include(cid2)
 
-      expect(transfer).to.be.an.instanceOf(Array)
-      expect(transfer).to.have.property('length', 3)
+      expect(transfer).to.be.an.instanceOf(Set)
+      expect(transfer).to.have.property('size', 3)
       expect(transfer).to.include(cid1.multihash.bytes.buffer)
       expect(transfer).to.include(cid2.multihash.bytes.buffer)
       expect(transfer).to.include(hi.buffer)
diff --git a/packages/ipfs-message-port-protocol/test/util.js b/packages/ipfs-message-port-protocol/test/util.js
index bc32d6653c..a1a93670c0 100644
--- a/packages/ipfs-message-port-protocol/test/util.js
+++ b/packages/ipfs-message-port-protocol/test/util.js
@@ -15,7 +15,7 @@ export const ipc = () => {
   /**
    * @template T
    * @param {T} data
-   * @param {Transferable[]} [transfer]
+   * @param {Iterable<Transferable>} [transfer]
    * @returns {Promise<T>}
    */
   const ipcMove = async (data, transfer = []) => {
diff --git a/packages/ipfs-message-port-server/src/block.js b/packages/ipfs-message-port-server/src/block.js
index 94e57aa903..6f6115090d 100644
--- a/packages/ipfs-message-port-server/src/block.js
+++ b/packages/ipfs-message-port-server/src/block.js
@@ -23,7 +23,7 @@ export class BlockService {
   /**
    * @typedef {Object} GetResult
    * @property {Uint8Array} block
-   * @property {Transferable[]} transfer
+   * @property {Set<Transferable>} transfer
    *
    * @typedef {Object} GetQuery
    * @property {EncodedCID} cid
@@ -36,15 +36,15 @@ export class BlockService {
   async get (query) {
     const cid = decodeCID(query.cid)
     const block = await this.ipfs.block.get(cid, query)
-    /** @type {Transferable[]} */
-    const transfer = []
+    /** @type {Set<Transferable>} */
+    const transfer = new Set()
     return { transfer, block: encodeBlock(block, transfer) }
   }
 
   /**
    * @typedef {Object} PutResult
    * @property {EncodedCID} cid
-   * @property {Transferable[]} transfer
+   * @property {Set<Transferable>} transfer
    *
    * @typedef {Object} PutQuery
    * @property {Uint8Array} block
@@ -58,9 +58,8 @@ export class BlockService {
   async put (query) {
     const input = query.block
     const result = await this.ipfs.block.put(input, query)
-
-    /** @type {Transferable[]} */
-    const transfer = []
+    /** @type {Set<Transferable>} */
+    const transfer = new Set()
 
     return { transfer, cid: encodeCID(result, transfer) }
   }
@@ -78,8 +77,8 @@ export class BlockService {
    * @returns {Promise<EncodedRmResult[]>}
    */
   async rm (query) {
-    /** @type {Transferable[]} */
-    const transfer = []
+    /** @type {Set<Transferable>} */
+    const transfer = new Set()
     const result = await all(
       this.ipfs.block.rm(query.cids.map(decodeCID), query)
     )
@@ -113,7 +112,7 @@ export class BlockService {
  * @param {Object} entry
  * @param {CID} entry.cid
  * @param {Error|void} [entry.error]
- * @param {Transferable[]} transfer
+ * @param {Set<Transferable>} transfer
  */
 const encodeRmEntry = (entry, transfer) => {
   const cid = encodeCID(entry.cid, transfer)
diff --git a/packages/ipfs-message-port-server/src/core.js b/packages/ipfs-message-port-server/src/core.js
index 8a147e7910..65f27ef387 100644
--- a/packages/ipfs-message-port-server/src/core.js
+++ b/packages/ipfs-message-port-server/src/core.js
@@ -247,8 +247,8 @@ const matchInput = (input, decode) => {
  * @param {AsyncIterable<AddResult>} out
  */
 const encodeAddAllResult = out => {
-  /** @type {Transferable[]} */
-  const transfer = []
+  /** @type {Set<Transferable>} */
+  const transfer = new Set()
   return {
     data: encodeIterable(out, encodeFileOutput, transfer),
     transfer
@@ -259,8 +259,8 @@ const encodeAddAllResult = out => {
  * @param {AddResult} out
  */
 const encodeAddResult = out => {
-  /** @type {Transferable[]} */
-  const transfer = []
+  /** @type {Set<Transferable>} */
+  const transfer = new Set()
   return {
     data: encodeFileOutput(out, transfer),
     transfer
@@ -271,8 +271,8 @@ const encodeAddResult = out => {
  * @param {AsyncIterable<Uint8Array>} content
  */
 const encodeCatResult = content => {
-  /** @type {Transferable[]} */
-  const transfer = []
+  /** @type {Set<Transferable>} */
+  const transfer = new Set()
   return { data: encodeIterable(content, moveBuffer, transfer), transfer }
 }
 
@@ -280,8 +280,8 @@ const encodeCatResult = content => {
  * @param {AsyncIterable<IPFSEntry>} entries
  */
 const encodeLsResult = entries => {
-  /** @type {Transferable[]} */
-  const transfer = []
+  /** @type {Set<Transferable>} */
+  const transfer = new Set()
   return { data: encodeIterable(entries, encodeLsEntry, transfer), transfer }
 }
 
@@ -302,17 +302,17 @@ const encodeLsEntry = ({ name, path, size, cid, type, mode, mtime }) => ({
  * Adds underlying `ArrayBuffer` to the transfer list.
  *
  * @param {Uint8Array} buffer
- * @param {Transferable[]} transfer
+ * @param {Set<Transferable>} transfer
  * @returns {Uint8Array}
  */
 const moveBuffer = (buffer, transfer) => {
-  transfer.push(buffer.buffer)
+  transfer.add(buffer.buffer)
   return buffer
 }
 
 /**
  * @param {AddResult} file
- * @param {Transferable[]} _transfer
+ * @param {Set<Transferable>} _transfer
  */
 const encodeFileOutput = (file, _transfer) => ({
   ...file,
diff --git a/packages/ipfs-message-port-server/src/dag.js b/packages/ipfs-message-port-server/src/dag.js
index 852f414f15..6ddc340c26 100644
--- a/packages/ipfs-message-port-server/src/dag.js
+++ b/packages/ipfs-message-port-server/src/dag.js
@@ -34,7 +34,7 @@ export class DAGService {
 
   /**
    * @typedef {Object} EncodedGetResult
-   * @property {Transferable[]} transfer
+   * @property {Set<Transferable>} transfer
    * @property {string} [remainderPath]
    * @property {EncodedDAGNode} value
    *
@@ -60,8 +60,8 @@ export class DAGService {
       }
     )
 
-    /** @type {Transferable[]} */
-    const transfer = []
+    /** @type {Set<Transferable>} */
+    const transfer = new Set()
     return { remainderPath, value: encodeNode(value, transfer), transfer }
   }
 
diff --git a/packages/ipfs-message-port-server/src/files.js b/packages/ipfs-message-port-server/src/files.js
index 3e0a7c51f4..77dbb1510b 100644
--- a/packages/ipfs-message-port-server/src/files.js
+++ b/packages/ipfs-message-port-server/src/files.js
@@ -26,8 +26,8 @@ export class FilesService {
    */
   async stat (input) {
     const stat = await this.ipfs.files.stat(input.path, input)
-    /** @type {Transferable[]} */
-    const transfer = []
+    /** @type {Set<Transferable>} */
+    const transfer = new Set()
     return { stat: { ...stat, cid: encodeCID(stat.cid, transfer) }, transfer }
   }
 }
diff --git a/packages/ipfs-message-port-server/src/server.js b/packages/ipfs-message-port-server/src/server.js
index 77ee644c10..785568c047 100644
--- a/packages/ipfs-message-port-server/src/server.js
+++ b/packages/ipfs-message-port-server/src/server.js
@@ -32,6 +32,11 @@ import { encodeError } from 'ipfs-message-port-protocol/error'
  * @typedef {import('ipfs-message-port-protocol/src/rpc').ServiceQuery<T>} ServiceQuery
  */
 
+/**
+ * @template T
+ * @typedef {import('ipfs-message-port-protocol/src/rpc').Return<T>} Return
+ */
+
 /**
  * @template T
  * @typedef {import('ipfs-message-port-protocol/src/rpc').RPCQuery<T>} RPCQuery
@@ -65,7 +70,7 @@ import { encodeError } from 'ipfs-message-port-protocol/error'
 
 /**
  * @typedef {Object} TransferOptions
- * @property {Transferable[]} [transfer]
+ * @property {Set<Transferable>} [transfer]
  */
 
 /**
@@ -97,6 +102,7 @@ export class Query {
    * @param {Inn<T>} input
    */
   constructor (namespace, method, input) {
+    /** @type {Return<any>} */
     this.result = new Promise((resolve, reject) => {
       this.succeed = resolve
       this.fail = reject
@@ -210,7 +216,9 @@ export class Server {
     if (!query.signal.aborted) {
       try {
         const value = await query.result
-        const transfer = [...new Set(value.transfer || [])]
+        const transfer = value.transfer
+
+        // Don't need the transfer value in the result
         delete value.transfer
 
         port.postMessage(
diff --git a/packages/ipfs-message-port-server/test/transfer.spec.js b/packages/ipfs-message-port-server/test/transfer.spec.js
index fcbef72e95..e60ae4caf9 100644
--- a/packages/ipfs-message-port-server/test/transfer.spec.js
+++ b/packages/ipfs-message-port-server/test/transfer.spec.js
@@ -14,7 +14,7 @@ describe('Server', function () {
     const cid = CID.parse('QmSnuWmxptJZdLJpKRarxBMS2Ju2oANVrgbr2xWbie9b2D')
 
     return new Promise((resolve, reject) => {
-      const channel = process.browser
+      const channel = globalThis.MessageChannel
         ? new globalThis.MessageChannel()
         : new MessageChannel()
 
@@ -29,7 +29,7 @@ describe('Server', function () {
 
       const service = new IPFSService()
       const server = new Server(service)
-      const transfer = []
+      const transfer = new Set()
 
       server.run = a => a
       server.handleQuery(