diff --git a/docs/schema/V1/swagger.verified.json b/docs/schema/V1/swagger.verified.json
index bcde68739..37754ac46 100644
--- a/docs/schema/V1/swagger.verified.json
+++ b/docs/schema/V1/swagger.verified.json
@@ -231,7 +231,7 @@
             "type": "string"
           },
           "process": {
-            "description": "Optional process identifier used to indicate a business process this dialog belongs to ",
+            "description": "Optional process identifier used to indicate a business process this dialog belongs to",
             "nullable": true,
             "type": "string"
           },
@@ -262,7 +262,7 @@
             ]
           },
           "systemLabel": {
-            "description": "Set the system label of the dialog Migration purposes ",
+            "description": "Set the system label of the dialog Migration purposes",
             "nullable": true,
             "oneOf": [
               {
@@ -751,6 +751,13 @@
             },
             "type": "array"
           },
+          "id": {
+            "description": "A self-defined UUIDv7 may be provided to support idempotent creation of transmission attachments. If not provided, a new UUIDv7 will be generated.",
+            "example": "01913cd5-784f-7d3b-abef-4c77b1f0972d",
+            "format": "guid",
+            "nullable": true,
+            "type": "string"
+          },
           "urls": {
             "description": "The URLs associated with the attachment, each referring to a different representation of the attachment.",
             "items": {
@@ -4089,6 +4096,13 @@
             },
             "type": "array"
           },
+          "id": {
+            "description": "A self-defined UUIDv7 may be provided to support idempotent creation of transmission attachments. If not provided, a new UUIDv7 will be generated.",
+            "example": "01913cd5-784f-7d3b-abef-4c77b1f0972d",
+            "format": "guid",
+            "nullable": true,
+            "type": "string"
+          },
           "urls": {
             "description": "The URLs associated with the attachment, each referring to a different representation of the attachment.",
             "items": {
diff --git a/src/Digdir.Domain.Dialogporten.Application/Features/V1/ServiceOwner/Dialogs/Commands/Create/CreateDialogCommand.cs b/src/Digdir.Domain.Dialogporten.Application/Features/V1/ServiceOwner/Dialogs/Commands/Create/CreateDialogCommand.cs
index f6e4beebd..e8dd6fc0e 100644
--- a/src/Digdir.Domain.Dialogporten.Application/Features/V1/ServiceOwner/Dialogs/Commands/Create/CreateDialogCommand.cs
+++ b/src/Digdir.Domain.Dialogporten.Application/Features/V1/ServiceOwner/Dialogs/Commands/Create/CreateDialogCommand.cs
@@ -117,5 +117,11 @@ private async Task EnsureNoExistingUserDefinedIds(DialogEntity dialog, Cancellat
         {
             _domainContext.AddError(DomainFailure.EntityExists<DialogTransmission>(existingTransmissionIds));
         }
+
+        var existingTransmissionAttachmentIds = await _db.GetExistingIds(dialog.Transmissions.SelectMany(t => t.Attachments), cancellationToken);
+        if (existingTransmissionAttachmentIds.Count != 0)
+        {
+            _domainContext.AddError(DomainFailure.EntityExists<DialogTransmissionAttachment>(existingTransmissionAttachmentIds));
+        }
     }
 }
diff --git a/src/Digdir.Domain.Dialogporten.Application/Features/V1/ServiceOwner/Dialogs/Commands/Create/CreateDialogCommandValidator.cs b/src/Digdir.Domain.Dialogporten.Application/Features/V1/ServiceOwner/Dialogs/Commands/Create/CreateDialogCommandValidator.cs
index 113924872..a9da828ef 100644
--- a/src/Digdir.Domain.Dialogporten.Application/Features/V1/ServiceOwner/Dialogs/Commands/Create/CreateDialogCommandValidator.cs
+++ b/src/Digdir.Domain.Dialogporten.Application/Features/V1/ServiceOwner/Dialogs/Commands/Create/CreateDialogCommandValidator.cs
@@ -277,6 +277,10 @@ public CreateDialogTransmissionAttachmentDtoValidator(
         IValidator<IEnumerable<LocalizationDto>> localizationsValidator,
         IValidator<CreateDialogTransmissionAttachmentUrlDto> urlValidator)
     {
+        RuleFor(x => x.Id)
+            .IsValidUuidV7()
+            .UuidV7TimestampIsInPast();
+
         RuleFor(x => x.DisplayName)
             .SetValidator(localizationsValidator);
         RuleFor(x => x.Urls)
diff --git a/src/Digdir.Domain.Dialogporten.Application/Features/V1/ServiceOwner/Dialogs/Commands/Create/CreateDialogDto.cs b/src/Digdir.Domain.Dialogporten.Application/Features/V1/ServiceOwner/Dialogs/Commands/Create/CreateDialogDto.cs
index 3722710c3..3d706fe9f 100644
--- a/src/Digdir.Domain.Dialogporten.Application/Features/V1/ServiceOwner/Dialogs/Commands/Create/CreateDialogDto.cs
+++ b/src/Digdir.Domain.Dialogporten.Application/Features/V1/ServiceOwner/Dialogs/Commands/Create/CreateDialogDto.cs
@@ -67,7 +67,7 @@ public class CreateDialogDto
     public DateTimeOffset? DueAt { get; set; }
 
     /// <summary>
-    /// Optional process identifier used to indicate a business process this dialog belongs to 
+    /// Optional process identifier used to indicate a business process this dialog belongs to
     /// </summary>
     public string? Process { get; set; }
     /// <summary>
@@ -104,7 +104,7 @@ public class CreateDialogDto
     public DialogStatus.Values Status { get; set; }
 
     /// <summary>
-    /// Set the system label of the dialog Migration purposes 
+    /// Set the system label of the dialog Migration purposes
     /// </summary>
     public SystemLabel.Values? SystemLabel { get; set; }
     /// <summary>
@@ -527,6 +527,12 @@ public sealed class CreateDialogDialogAttachmentUrlDto
 
 public sealed class CreateDialogTransmissionAttachmentDto
 {
+    /// <summary>
+    /// A self-defined UUIDv7 may be provided to support idempotent creation of transmission attachments. If not provided, a new UUIDv7 will be generated.
+    /// </summary>
+    /// <example>01913cd5-784f-7d3b-abef-4c77b1f0972d</example>
+    public Guid? Id { get; set; }
+
     /// <summary>
     /// The display name of the attachment that should be used in GUIs.
     /// </summary>
diff --git a/src/Digdir.Domain.Dialogporten.Application/Features/V1/ServiceOwner/Dialogs/Commands/Update/MappingProfile.cs b/src/Digdir.Domain.Dialogporten.Application/Features/V1/ServiceOwner/Dialogs/Commands/Update/MappingProfile.cs
index bd3591f2f..aa7d94d00 100644
--- a/src/Digdir.Domain.Dialogporten.Application/Features/V1/ServiceOwner/Dialogs/Commands/Update/MappingProfile.cs
+++ b/src/Digdir.Domain.Dialogporten.Application/Features/V1/ServiceOwner/Dialogs/Commands/Update/MappingProfile.cs
@@ -81,8 +81,7 @@ public MappingProfile()
             .ForMember(dest => dest.ActorType, opt => opt.Ignore())
             .ForMember(dest => dest.ActorTypeId, opt => opt.MapFrom(src => src.ActorType));
 
-        CreateMap<UpdateDialogTransmissionAttachmentDto, DialogTransmissionAttachment>()
-            .ForMember(x => x.Id, opt => opt.Ignore());
+        CreateMap<UpdateDialogTransmissionAttachmentDto, DialogTransmissionAttachment>();
 
         CreateMap<UpdateDialogTransmissionAttachmentUrlDto, AttachmentUrl>()
             .ForMember(x => x.Id, opt => opt.Ignore())
diff --git a/src/Digdir.Domain.Dialogporten.Application/Features/V1/ServiceOwner/Dialogs/Commands/Update/UpdateDialogCommand.cs b/src/Digdir.Domain.Dialogporten.Application/Features/V1/ServiceOwner/Dialogs/Commands/Update/UpdateDialogCommand.cs
index 4aaac0441..421857dba 100644
--- a/src/Digdir.Domain.Dialogporten.Application/Features/V1/ServiceOwner/Dialogs/Commands/Update/UpdateDialogCommand.cs
+++ b/src/Digdir.Domain.Dialogporten.Application/Features/V1/ServiceOwner/Dialogs/Commands/Update/UpdateDialogCommand.cs
@@ -282,6 +282,17 @@ private async Task AppendTransmission(DialogEntity dialog, UpdateDialogDto dto,
             return;
         }
 
+        var newTransmissionAttachments = newDialogTransmissions
+            .SelectMany(x => x.Attachments)
+            .ToList();
+
+        var existingTransmissionAttachmentIds = await _db.GetExistingIds(newTransmissionAttachments, cancellationToken);
+        if (existingTransmissionAttachmentIds.Count != 0)
+        {
+            _domainContext.AddError(DomainFailure.EntityExists<DialogTransmissionAttachment>(existingTransmissionAttachmentIds));
+            return;
+        }
+
         dialog.Transmissions.AddRange(newDialogTransmissions);
         // Tell ef explicitly to add transmissions as new to the database.
         _db.DialogTransmissions.AddRange(newDialogTransmissions);
diff --git a/src/Digdir.Domain.Dialogporten.Application/Features/V1/ServiceOwner/Dialogs/Commands/Update/UpdateDialogCommandValidator.cs b/src/Digdir.Domain.Dialogporten.Application/Features/V1/ServiceOwner/Dialogs/Commands/Update/UpdateDialogCommandValidator.cs
index 1cf5b1c11..77cee4e86 100644
--- a/src/Digdir.Domain.Dialogporten.Application/Features/V1/ServiceOwner/Dialogs/Commands/Update/UpdateDialogCommandValidator.cs
+++ b/src/Digdir.Domain.Dialogporten.Application/Features/V1/ServiceOwner/Dialogs/Commands/Update/UpdateDialogCommandValidator.cs
@@ -114,6 +114,10 @@ public UpdateDialogTransmissionAttachmentDtoValidator(
         IValidator<IEnumerable<LocalizationDto>> localizationsValidator,
         IValidator<UpdateDialogTransmissionAttachmentUrlDto> urlValidator)
     {
+        RuleFor(x => x.Id)
+            .IsValidUuidV7()
+            .UuidV7TimestampIsInPast();
+
         RuleFor(x => x.DisplayName)
             .SetValidator(localizationsValidator);
         RuleFor(x => x.Urls)
diff --git a/src/Digdir.Domain.Dialogporten.Application/Features/V1/ServiceOwner/Dialogs/Commands/Update/UpdateDialogDto.cs b/src/Digdir.Domain.Dialogporten.Application/Features/V1/ServiceOwner/Dialogs/Commands/Update/UpdateDialogDto.cs
index 8182b048d..1c388a844 100644
--- a/src/Digdir.Domain.Dialogporten.Application/Features/V1/ServiceOwner/Dialogs/Commands/Update/UpdateDialogDto.cs
+++ b/src/Digdir.Domain.Dialogporten.Application/Features/V1/ServiceOwner/Dialogs/Commands/Update/UpdateDialogDto.cs
@@ -510,6 +510,12 @@ public sealed class UpdateDialogDialogAttachmentUrlDto
 
 public sealed class UpdateDialogTransmissionAttachmentDto
 {
+    /// <summary>
+    /// A self-defined UUIDv7 may be provided to support idempotent creation of transmission attachments. If not provided, a new UUIDv7 will be generated.
+    /// </summary>
+    /// <example>01913cd5-784f-7d3b-abef-4c77b1f0972d</example>
+    public Guid? Id { get; set; }
+
     /// <summary>
     /// The display name of the attachment that should be used in GUIs.
     /// </summary>