Fix that you can't add a content part when it's already added as a Na…

…med Part (#16495)

src/OrchardCore.Modules/OrchardCore.ContentTypes/Controllers/AdminController.cs

@@ -249,7 +249,7 @@ public async Task<ActionResult> AddPartsTo(string id)
                 return NotFound();
             }
 
-            var typePartNames = new HashSet<string>(typeViewModel.TypeDefinition.Parts.Select(p => p.PartDefinition.Name));
+            var typePartNames = new HashSet<string>(typeViewModel.TypeDefinition.Parts.Select(p => p.IsNamedPart() ? p.Name : p.PartDefinition.Name));
 
             var viewModel = new AddPartsViewModel
             {