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 7ac9ca025..5d7430510 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
@@ -183,9 +183,7 @@ private async Task AppendActivity(DialogEntity dialog, UpdateDialogDto dto, Canc
         var existingIds = await _db.GetExistingIds(newDialogActivities, cancellationToken);
         if (existingIds.Count != 0)
         {
-            _domainContext.AddError(
-                nameof(UpdateDialogDto.Activities),
-                $"Entity '{nameof(DialogActivity)}' with the following key(s) already exists: ({string.Join(", ", existingIds)}).");
+            _domainContext.AddError(DomainFailure.EntityExists<DialogActivity>(existingIds));
             return;
         }
 
@@ -257,6 +255,7 @@ private async Task AppendTransmission(DialogEntity dialog, UpdateDialogDto dto,
         if (existingIds.Count != 0)
         {
             _domainContext.AddError(DomainFailure.EntityExists<DialogTransmission>(existingIds));
+            return;
         }
 
         dialog.Transmissions.AddRange(newDialogTransmissions);
diff --git a/src/Digdir.Tool.Dialogporten.GenerateFakeData/DialogGenerator.cs b/src/Digdir.Tool.Dialogporten.GenerateFakeData/DialogGenerator.cs
index 68072281e..15ffde70e 100644
--- a/src/Digdir.Tool.Dialogporten.GenerateFakeData/DialogGenerator.cs
+++ b/src/Digdir.Tool.Dialogporten.GenerateFakeData/DialogGenerator.cs
@@ -7,6 +7,7 @@
 using Digdir.Domain.Dialogporten.Domain.Dialogs.Entities;
 using Digdir.Domain.Dialogporten.Domain.Dialogs.Entities.Actions;
 using Digdir.Domain.Dialogporten.Domain.Dialogs.Entities.Activities;
+using Digdir.Domain.Dialogporten.Domain.Dialogs.Entities.Transmissions;
 using Digdir.Domain.Dialogporten.Domain.Http;
 using Medo;
 
@@ -227,6 +228,22 @@ private static int CalculateControlDigit(string input, int[] weights)
     public static CreateDialogDialogActivityDto GenerateFakeDialogActivity(DialogActivityType.Values? type = null)
         => GenerateFakeDialogActivities(1, type)[0];
 
+    public static List<CreateDialogDialogTransmissionDto> GenerateFakeDialogTransmissions(int? count = null,
+        DialogTransmissionType.Values? type = null)
+    {
+        return new Faker<CreateDialogDialogTransmissionDto>()
+            .RuleFor(o => o.Id, _ => Uuid7.NewUuid7().ToGuid(true))
+            .RuleFor(o => o.CreatedAt, f => f.Date.Past())
+            .RuleFor(o => o.Type, f => type ?? f.PickRandom<DialogTransmissionType.Values>())
+            .RuleFor(o => o.Sender, _ => new() { ActorType = ActorType.Values.ServiceOwner })
+            .RuleFor(o => o.Content, _ => new()
+            {
+                Title = new() { Value = GenerateFakeLocalizations(1) },
+                Summary = new() { Value = GenerateFakeLocalizations(3) }
+            })
+            .Generate(count ?? new Randomizer().Number(1, 4));
+    }
+
     public static List<CreateDialogDialogActivityDto> GenerateFakeDialogActivities(int? count = null, DialogActivityType.Values? type = null)
     {
         return new Faker<CreateDialogDialogActivityDto>()
diff --git a/tests/Digdir.Domain.Dialogporten.Application.Integration.Tests/Features/V1/ServiceOwner/Dialogs/Commands/UpdateDialogTests.cs b/tests/Digdir.Domain.Dialogporten.Application.Integration.Tests/Features/V1/ServiceOwner/Dialogs/Commands/UpdateDialogTests.cs
index c1dafe4bd..167555250 100644
--- a/tests/Digdir.Domain.Dialogporten.Application.Integration.Tests/Features/V1/ServiceOwner/Dialogs/Commands/UpdateDialogTests.cs
+++ b/tests/Digdir.Domain.Dialogporten.Application.Integration.Tests/Features/V1/ServiceOwner/Dialogs/Commands/UpdateDialogTests.cs
@@ -4,6 +4,7 @@
 using Digdir.Domain.Dialogporten.Application.Integration.Tests.Common;
 using Digdir.Domain.Dialogporten.Domain.Actors;
 using Digdir.Domain.Dialogporten.Domain.Dialogs.Entities.Activities;
+using Digdir.Domain.Dialogporten.Domain.Dialogs.Entities.Transmissions;
 using Digdir.Tool.Dialogporten.GenerateFakeData;
 using FluentAssertions;
 
@@ -42,6 +43,74 @@ public async Task New_Activity_Should_Be_Able_To_Refer_To_Old_Activity()
         result.Should().NotBeNull();
     }
 
+    [Fact]
+    public async Task Cannot_Include_Old_Activities_To_UpdateCommand()
+    {
+        // Arrange
+        var (_, createCommandResponse) = await GenerateDialogWithActivity();
+        var getDialogQuery = new GetDialogQuery { DialogId = createCommandResponse.AsT0.Value };
+        var getDialogDto = await Application.Send(getDialogQuery);
+
+        var mapper = Application.GetMapper();
+        var updateDialogDto = mapper.Map<UpdateDialogDto>(getDialogDto.AsT0);
+
+        // Ref. old activity
+        updateDialogDto.Activities.Add(new UpdateDialogDialogActivityDto
+        {
+            Id = getDialogDto.AsT0.Activities.First().Id,
+            Type = DialogActivityType.Values.DialogCreated,
+            PerformedBy = new UpdateDialogDialogActivityPerformedByActorDto
+            {
+                ActorType = ActorType.Values.ServiceOwner
+            }
+        });
+
+        // Act
+        var updateResponse = await Application.Send(new UpdateDialogCommand { Id = createCommandResponse.AsT0.Value, Dto = updateDialogDto });
+
+        // Assert
+        updateResponse.TryPickT5(out var domainError, out _).Should().BeTrue();
+        domainError.Should().NotBeNull();
+        domainError.Errors.Should().Contain(e => e.ErrorMessage.Contains("already exists"));
+    }
+
+    [Fact]
+    public async Task Cannot_Include_Old_Transmissions_In_UpdateCommand()
+    {
+        // Arrange
+        var createDialogCommand = DialogGenerator.GenerateSimpleFakeDialog();
+        var existingTransmission = DialogGenerator.GenerateFakeDialogTransmissions(count: 1).First();
+        createDialogCommand.Transmissions.Add(existingTransmission);
+        var createCommandResponse = await Application.Send(createDialogCommand);
+
+        var getDialogQuery = new GetDialogQuery { DialogId = createCommandResponse.AsT0.Value };
+        var getDialogDto = await Application.Send(getDialogQuery);
+
+        var mapper = Application.GetMapper();
+        var updateDialogDto = mapper.Map<UpdateDialogDto>(getDialogDto.AsT0);
+
+        // Ref. old transmission
+        updateDialogDto.Transmissions.Add(new UpdateDialogDialogTransmissionDto
+        {
+            Id = existingTransmission.Id,
+            Type = DialogTransmissionType.Values.Information,
+            Sender = new() { ActorType = ActorType.Values.ServiceOwner },
+            Content = new()
+            {
+                Title = new() { Value = DialogGenerator.GenerateFakeLocalizations(3) },
+                Summary = new() { Value = DialogGenerator.GenerateFakeLocalizations(3) }
+            }
+        });
+
+        // Act
+        var updateResponse = await Application.Send(new UpdateDialogCommand { Id = createCommandResponse.AsT0.Value, Dto = updateDialogDto });
+
+        // Assert
+        updateResponse.TryPickT5(out var domainError, out _).Should().BeTrue();
+        domainError.Should().NotBeNull();
+        domainError.Errors.Should().Contain(e => e.ErrorMessage.Contains("already exists"));
+    }
+
     private async Task<(CreateDialogCommand, CreateDialogResult)> GenerateDialogWithActivity()
     {
         var createDialogCommand = DialogGenerator.GenerateSimpleFakeDialog();