I guess the DoubleClicking events in the RadTreeView do not fit in your scenario. However, usually when a file in Windows is Selected, then after another click and mouse up the file enters an Edit Mode. So you can use the MouseLeftButtonUp event of the RadTreeViewItems and check whether the item is already selected.
By default this method is handled internally by RadTreeViewItem but you are able to use the AddHandler and its boolean parameter handledEvents set tot true.
Hope this gives you the right direction.
the Telerik team