diff --git a/CHANGES b/CHANGES
index 2f62d8dc5..381ad1220 100644
--- a/CHANGES
+++ b/CHANGES
@@ -6,6 +6,9 @@
 
 ====== Fixes ======
 
+  * Use is_blocking in dup and dup2 to fix ENOTSOCK on Windows.
+    (#869, Antonin Décimo)
+
   * Support IPv6 socketpair on Windows (#870, #876, Antonin Décimo, David Allsopp).
 
   * Lwt_unix.lstat was incorrectly calling Unix.stat on Win32. Fixes
diff --git a/src/unix/lwt_unix.cppo.ml b/src/unix/lwt_unix.cppo.ml
index 5cfcba277..4c59e2671 100644
--- a/src/unix/lwt_unix.cppo.ml
+++ b/src/unix/lwt_unix.cppo.ml
@@ -1242,13 +1242,8 @@ let dup ch =
     set_flags = ch.set_flags;
     blocking =
       if ch.set_flags then
-        lazy(Lazy.force ch.blocking >>= function
-        | true ->
-          Unix.clear_nonblock fd;
-          Lwt.return_true
-        | false ->
-          Unix.set_nonblock fd;
-          Lwt.return_false)
+        lazy(Lazy.force ch.blocking >>= function blocking ->
+               Lazy.force (is_blocking ~blocking fd))
       else
         ch.blocking;
     event_readable = None;
@@ -1263,13 +1258,8 @@ let dup2 ch1 ch2 =
   ch2.set_flags <- ch1.set_flags;
   ch2.blocking <- (
     if ch2.set_flags then
-      lazy(Lazy.force ch1.blocking >>= function
-      | true ->
-        Unix.clear_nonblock ch2.fd;
-        Lwt.return_true
-      | false ->
-        Unix.set_nonblock ch2.fd;
-        Lwt.return_false)
+      lazy(Lazy.force ch1.blocking >>= function blocking ->
+             Lazy.force (is_blocking ~blocking ch2.fd))
     else
       ch1.blocking
   )
diff --git a/test/unix/test_lwt_unix.cppo.ml b/test/unix/test_lwt_unix.cppo.ml
index e758669cc..0ed248dd5 100644
--- a/test/unix/test_lwt_unix.cppo.ml
+++ b/test/unix/test_lwt_unix.cppo.ml
@@ -1173,6 +1173,82 @@ let pread_tests ~blocking =
       Lwt.return_true);
 ]
 
+let dup_tests ~blocking =
+  let test_file = test_filename "test_dup" in
+  let file_contents = "01234567890123456789" in
+  let len = String.length file_contents in
+  let buf = Bytes.make len '\x00' in
+  let blocking_string =
+    if blocking then
+      " blocking"
+    else
+      " nonblocking"
+  in
+  [
+  test ~sequential:true ("dup on socket" ^ blocking_string)
+    (fun () ->
+      let s1, s2 =
+        if Sys.win32 then Lwt_unix.socketpair Unix.PF_INET6 Unix.SOCK_STREAM 0
+        else Lwt_unix.socketpair Unix.PF_UNIX Unix.SOCK_STREAM 0
+      in
+      if not blocking then Lwt_unix.set_blocking ~set_flags:false s1 false;
+      let s1' = Lwt_unix.dup s1 in
+      Lwt_unix.blocking s1
+      >>= fun s1_is_blocking ->
+      Lwt_unix.blocking s1'
+      >>= fun s1'_is_blocking ->
+      assert(s1_is_blocking = s1'_is_blocking);
+      Lwt_unix.write_string s1 file_contents 0 len
+      >>= fun n ->
+      assert(n = len);
+      Lwt_unix.read s2 buf 0 len
+      >>= fun n ->
+      assert(n = len);
+      let read = Bytes.to_string buf in
+      assert(read = file_contents);
+      Lwt_unix.write_string s1' file_contents 0 len
+      >>= fun n ->
+      assert(n = len);
+      Lwt_unix.read s2 buf 0 len
+      >>= fun n ->
+      assert(n = len);
+      let read = Bytes.to_string buf in
+      assert(read = file_contents);
+      Lwt_list.iter_p Lwt_unix.close [s1; s1'; s2] >>= fun () ->
+      Lwt.return_true);
+
+    test ~sequential:true ("dup on file" ^ blocking_string)
+    (fun () ->
+      Lwt_unix.openfile test_file [O_RDWR; O_TRUNC; O_CREAT] 0o666
+      >>= fun fd ->
+      if not blocking then Lwt_unix.set_blocking ~set_flags:false fd false;
+      let fd' = Lwt_unix.dup fd in
+      Lwt_unix.blocking fd
+      >>= fun fd_is_blocking ->
+      Lwt_unix.blocking fd'
+      >>= fun fd'_is_blocking ->
+      assert(fd_is_blocking = fd'_is_blocking);
+      Lwt_unix.write_string fd file_contents 0 len
+      >>= fun n ->
+      assert(n = len);
+      Lwt_unix.lseek fd 0 Lwt_unix.SEEK_SET >>= fun _pos ->
+      let buf = Bytes.make (String.length file_contents) '\x00' in
+      Lwt_unix.read fd buf 0 (String.length file_contents) >>= fun n ->
+      assert(n = (String.length file_contents));
+      let read = Bytes.to_string buf in
+      assert (read = file_contents);
+      Lwt_unix.write_string fd' file_contents 0 len
+      >>= fun n ->
+      assert(n = len);
+      Lwt_unix.lseek fd' 0 Lwt_unix.SEEK_SET >>= fun _pos ->
+      let buf = Bytes.make (String.length file_contents) '\x00' in
+      Lwt_unix.read fd' buf 0 (String.length file_contents) >>= fun n ->
+      assert(n = (String.length file_contents));
+      let read = Bytes.to_string buf in
+      assert (read = file_contents);
+      Lwt.return_true);
+]
+
 let suite =
   suite "lwt_unix"
     (wait_tests @
@@ -1188,5 +1264,7 @@ let suite =
      lwt_preemptive_tests @
      lwt_user_tests @
      pread_tests ~blocking:true @
-     pread_tests ~blocking:false
+     pread_tests ~blocking:false @
+     dup_tests ~blocking:true @
+     dup_tests ~blocking:false
     )