Prevented created of duplicate children

If the `ITopicMappingService` returns an existing obect (e.g., if the object is cached, or if the mapping service followed relationships) then adding children could result in duplicates. This is a _bit_ of a leaky abstraction since implementers shouldn't need to be aware of whether the `ITopicMappingService` is caching. That said, this is a bit of an edge case since typically the `ITopicMappingService` adds children, so the extra level of caution is warranted regardless.

Author: JeremyCaney

Ignia.Topics.Web.Mvc/Controllers/LayoutController.cs

@@ -189,7 +189,7 @@ protected T AddNestedTopics(
         return null as T;
       }
       var viewModel = _topicMappingService.Map<T>(sourceTopic, Relationships.None);
-      if (tiers >= 0 && (allowPageGroups || !sourceTopic.ContentType.Equals("PageGroup"))) {
+      if (tiers >= 0 && (allowPageGroups || !sourceTopic.ContentType.Equals("PageGroup")) && viewModel.Children.Count == 0) {
         foreach (var topic in sourceTopic.Children.Sorted.Where(t => t.IsVisible())) {
           viewModel.Children.Add(
             AddNestedTopics(