diff --git a/ILSpy.Tests/AssemblyList/ManageAssemblyListsNameValidationTests.cs b/ILSpy.Tests/AssemblyList/ManageAssemblyListsNameValidationTests.cs
new file mode 100644
index 000000000..d7fbf7d63
--- /dev/null
+++ b/ILSpy.Tests/AssemblyList/ManageAssemblyListsNameValidationTests.cs
@@ -0,0 +1,104 @@
+// Copyright (c) 2026 Siegfried Pammer
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy of this
+// software and associated documentation files (the "Software"), to deal in the Software
+// without restriction, including without limitation the rights to use, copy, modify, merge,
+// publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons
+// to whom the Software is furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in all copies or
+// substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED,
+// INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR
+// PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE
+// FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
+// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+// DEALINGS IN THE SOFTWARE.
+
+using System;
+
+using Avalonia.Controls;
+using Avalonia.Headless.NUnit;
+using Avalonia.Threading;
+
+using AwesomeAssertions;
+
+using ICSharpCode.ILSpy;
+using ICSharpCode.ILSpy.AppEnv;
+using ICSharpCode.ILSpy.Views;
+
+using NUnit.Framework;
+
+namespace ICSharpCode.ILSpy.Tests;
+
+/// <summary>
+/// Manage Assembly Lists: a new list may not take the name of an existing one. The prompt has
+/// to say so and keep OK disabled - the operations behind it silently do nothing for a name
+/// that is taken, which reads as the dialog having accepted the name.
+/// </summary>
+[TestFixture]
+public class ManageAssemblyListsNameValidationTests
+{
+	static (ManageAssemblyListsDialog Dialog, string TakenName) DialogWithOneList()
+	{
+		var settingsService = AppComposition.Current.GetExport<SettingsService>();
+		var manager = settingsService.AssemblyListManager;
+		var taken = "List " + Guid.NewGuid().ToString("N");
+		manager.AddListIfNotExists(manager.CreateList(taken));
+		return (new ManageAssemblyListsDialog(settingsService), taken);
+	}
+
+	static (TextBox Name, Button Ok, TextBlock Message) Controls(CreateListDialog prompt)
+		=> (prompt.FindControl<TextBox>("ListNameBox")!,
+			prompt.FindControl<Button>("OkButton")!,
+			prompt.FindControl<TextBlock>("NameTakenText")!);
+
+	[AvaloniaTest]
+	public void Prompt_Rejects_The_Name_Of_An_Existing_List()
+	{
+		var (dialog, taken) = DialogWithOneList();
+		var prompt = dialog.CreatePrompt("New Assembly List");
+		prompt.Show();
+		try
+		{
+			var (name, ok, message) = Controls(prompt);
+
+			name.Text = taken;
+			Dispatcher.UIThread.RunJobs();
+			ok.IsEnabled.Should().BeFalse("the name is already in use");
+			message.IsVisible.Should().BeTrue("the user has to be told why OK is disabled");
+
+			name.Text = taken + " (2)";
+			Dispatcher.UIThread.RunJobs();
+			ok.IsEnabled.Should().BeTrue("the name is free");
+			message.IsVisible.Should().BeFalse();
+		}
+		finally
+		{
+			prompt.Close();
+			dialog.Close();
+		}
+	}
+
+	[AvaloniaTest]
+	public void Rename_Accepts_The_Name_The_List_Already_Has()
+	{
+		var (dialog, taken) = DialogWithOneList();
+		// Renaming a list to its own name is a no-op, not a collision with itself.
+		var prompt = dialog.CreatePrompt("Rename Assembly List", taken, allowedName: taken);
+		prompt.Show();
+		try
+		{
+			var (_, ok, message) = Controls(prompt);
+			Dispatcher.UIThread.RunJobs();
+			ok.IsEnabled.Should().BeTrue();
+			message.IsVisible.Should().BeFalse();
+		}
+		finally
+		{
+			prompt.Close();
+			dialog.Close();
+		}
+	}
+}
diff --git a/ILSpy/Views/CreateListDialog.axaml b/ILSpy/Views/CreateListDialog.axaml
index 58dc1806a..c4b8525cb 100644
--- a/ILSpy/Views/CreateListDialog.axaml
+++ b/ILSpy/Views/CreateListDialog.axaml
@@ -8,6 +8,8 @@
 	<Grid Margin="12,8" RowDefinitions="Auto,Auto,*,Auto">
 		<TextBlock Grid.Row="0" Text="Enter the name of the list:" Margin="0,0,0,6" />
 		<TextBox Grid.Row="1" Name="ListNameBox" Margin="0,4" />
+		<TextBlock Grid.Row="2" Name="NameTakenText" VerticalAlignment="Top"
+		           Foreground="Red" TextWrapping="Wrap" IsVisible="False" />
 		<StackPanel Grid.Row="3" Orientation="Horizontal" HorizontalAlignment="Right" Spacing="6">
 			<Button Name="OkButton" Content="OK" IsDefault="True" IsEnabled="False" MinWidth="72" />
 			<Button Name="CancelButton" Content="Cancel" IsCancel="True" MinWidth="72" />
diff --git a/ILSpy/Views/CreateListDialog.axaml.cs b/ILSpy/Views/CreateListDialog.axaml.cs
index 1b656fb82..88b0e9981 100644
--- a/ILSpy/Views/CreateListDialog.axaml.cs
+++ b/ILSpy/Views/CreateListDialog.axaml.cs
@@ -16,6 +16,8 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
+using System;
+
 using Avalonia.Controls;
 using Avalonia.Markup.Xaml;
 
@@ -32,25 +34,45 @@ namespace ICSharpCode.ILSpy.Views
 	{
 		TextBox listNameBox = null!;
 		Button okButton = null!;
+		TextBlock nameTakenText = null!;
+		Func<string, bool> isNameTaken = static _ => false;
 
 		public CreateListDialog()
 		{
 			InitializeComponent();
 			listNameBox = this.FindControl<TextBox>("ListNameBox")!;
 			okButton = this.FindControl<Button>("OkButton")!;
-			listNameBox.TextChanged += (_, _) => okButton.IsEnabled = !string.IsNullOrWhiteSpace(listNameBox.Text);
+			nameTakenText = this.FindControl<TextBlock>("NameTakenText")!;
+			nameTakenText.Text = Properties.Resources.ListExistsAlready;
+			listNameBox.TextChanged += (_, _) => Validate();
 			okButton.Click += (_, _) => Close(listNameBox.Text);
 			((Button)this.FindControl<Button>("CancelButton")!).Click += (_, _) => Close(null);
 		}
 
-		public CreateListDialog(string title, string? initialText = null) : this()
+		/// <param name="isNameTaken">
+		/// Decides whether the entered name is already in use. Every caller writes into the same
+		/// set of assembly-list names, so a name that is taken cannot be accepted: the operation
+		/// behind the prompt would do nothing at all and the dialog would look like it had
+		/// worked. OK stays disabled for as long as the name collides.
+		/// </param>
+		public CreateListDialog(string title, string? initialText = null, Func<string, bool>? isNameTaken = null) : this()
 		{
 			Title = title;
+			this.isNameTaken = isNameTaken ?? this.isNameTaken;
 			if (!string.IsNullOrEmpty(initialText))
 			{
 				listNameBox.Text = initialText;
 				listNameBox.SelectAll();
 			}
+			Validate();
+		}
+
+		void Validate()
+		{
+			var name = listNameBox.Text;
+			bool taken = !string.IsNullOrWhiteSpace(name) && isNameTaken(name);
+			nameTakenText.IsVisible = taken;
+			okButton.IsEnabled = !string.IsNullOrWhiteSpace(name) && !taken;
 		}
 
 		void InitializeComponent() => AvaloniaXamlLoader.Load(this);
diff --git a/ILSpy/Views/ManageAssemblyListsDialog.axaml.cs b/ILSpy/Views/ManageAssemblyListsDialog.axaml.cs
index a6d859ea9..8770c27b5 100644
--- a/ILSpy/Views/ManageAssemblyListsDialog.axaml.cs
+++ b/ILSpy/Views/ManageAssemblyListsDialog.axaml.cs
@@ -106,11 +106,18 @@ namespace ICSharpCode.ILSpy.Views
 		/// <summary>The lists control, exposed for tests to drive the selection.</summary>
 		internal ListBox ListsControl => listsBox;
 
-		async Task<string?> PromptAsync(string title, string? initialText = null)
-		{
-			var dlg = new CreateListDialog(title, initialText);
-			return await dlg.ShowDialog<string?>(this);
-		}
+		async Task<string?> PromptAsync(string title, string? initialText = null, string? allowedName = null)
+			=> await CreatePrompt(title, initialText, allowedName).ShowDialog<string?>(this);
+
+		/// <summary>
+		/// Builds the name prompt for one of the CRUD flows. <paramref name="allowedName"/> is the
+		/// name the flow may keep - the name of the list being renamed - which is not a collision
+		/// with itself. Separated from <see cref="PromptAsync"/> so it can be driven by tests
+		/// without a modal window.
+		/// </summary>
+		internal CreateListDialog CreatePrompt(string title, string? initialText = null, string? allowedName = null)
+			=> new CreateListDialog(title, initialText,
+				name => name != allowedName && manager.AssemblyLists.Contains(name));
 
 		async Task NewListAsync()
 		{
@@ -135,7 +142,7 @@ namespace ICSharpCode.ILSpy.Views
 		{
 			if (SelectedListName is not { } selected)
 				return;
-			var name = await PromptAsync("Rename Assembly List", selected);
+			var name = await PromptAsync("Rename Assembly List", selected, allowedName: selected);
 			if (string.IsNullOrWhiteSpace(name) || name == selected || manager.AssemblyLists.Contains(name))
 				return;
 			manager.RenameList(selected, name);