Add diagnostic for duplicate ICommand-s in base types

CommunityToolkit.Mvvm.SourceGenerators/Input/ICommandGenerator.Execute.cs

@@ -315,6 +315,25 @@ public static ImmutableArray<MemberDeclarationSyntax> GetSyntax(CommandInfo comm
         /// <returns>Whether or not <paramref name="methodSymbol"/> was unique within its containing type.</returns>
         private static bool IsCommandDefinitionUnique(IMethodSymbol methodSymbol, ImmutableArray<Diagnostic>.Builder diagnostics)
         {
+            // If a duplicate is present in any of the base types, always emit a diagnostic for the current method.
+            // That is, there is no need to check the order: we assume the priority is top-down in the type hierarchy.
+            // This check has to be done first, as otherwise there would always be a false positive for the current type.
+            foreach (ISymbol symbol in methodSymbol.ContainingType.BaseType?.GetAllMembers(methodSymbol.Name) ?? Enumerable.Empty<ISymbol>())
+            {
+                if (symbol is IMethodSymbol otherSymbol &&
+                    otherSymbol.HasAttributeWithFullyQualifiedName("global::CommunityToolkit.Mvvm.Input.ICommandAttribute"))
+                {
+                    diagnostics.Add(
+                        MultipleICommandMethodOverloadsError,
+                        methodSymbol,
+                        methodSymbol.ContainingType,
+                        methodSymbol);
+
+                    return false;
+                }
+            }
+
+            // Check for duplicates in the containing type for the annotated method
             foreach (ISymbol symbol in methodSymbol.ContainingType.GetMembers(methodSymbol.Name))
             {
                 if (symbol is IMethodSymbol otherSymbol &&

tests/CommunityToolkit.Mvvm.SourceGenerators.UnitTests/Test_SourceGeneratorsDiagnostics.cs

@@ -1105,6 +1105,34 @@ private void GreetUser(object value)
         VerifyGeneratedDiagnostics<ICommandGenerator>(source, "MVVMTK0023");
     }
 
+    [TestMethod]
+    public void MultipleICommandMethodOverloads_WithOverloadInBaseType()
+    {
+        string source = @"
+        using CommunityToolkit.Mvvm.Input;
+
+        namespace MyApp
+        {
+            public partial class BaseViewModel
+            {
+                [ICommand]
+                private void GreetUser()
+                {
+                }
+            }
+
+            public partial class SampleViewModel : BaseViewModel
+            {
+                [ICommand]
+                private void GreetUser(object value)
+                {
+                }
+            }
+        }";
+
+        VerifyGeneratedDiagnostics<ICommandGenerator>(source, "MVVMTK0023");
+    }
+
     [TestMethod]
     public void InvalidObservablePropertyError_Object()
     {