fix bug in SolutionFolder logic: projects inside solution folders were added twice in some cases

src/Main/SharpDevelop/Project/SolutionFolder.cs

@@ -82,6 +82,7 @@ namespace ICSharpCode.SharpDevelop.Project
 			
 			protected override void OnCollectionChanged(IReadOnlyCollection<ISolutionItem> removedItems, IReadOnlyCollection<ISolutionItem> addedItems)
 			{
+				if (folder.parentSolution.IsAncestorOf(folder)) {
 					using (folder.parentSolution.ReportBatch()) {
 						foreach (ISolutionItem item in removedItems) {
 							folder.parentSolution.ReportRemovedItem(item);
@@ -90,6 +91,7 @@ namespace ICSharpCode.SharpDevelop.Project
 							folder.parentSolution.ReportAddedItem(item);
 						}
 					}
+				}
 				base.OnCollectionChanged(removedItems, addedItems);
 				folder.parentSolution.IsDirty = true;
 			}