Repro Steps:

1) Create a new Project.
2) Navigate to the Resources Panel, and within the GlobalDictionary, create any type of Resource (I used a Color resource myself). This will make GlobalResources.xaml available.
3) Navigate to the top portion of the Resources panel, hover over GlobalDictionary.xaml, and press the (+) icon.
4) Under the "Merge Dictionary" submenu, select "GlobalDictionary". (Yes, we are merging GlobalDictionary into GlobalDictionary).

Observe how the application crashes.

Expected Results: The Dictionary you are merging into should not appear in the list of 'mergeable' dictionaries to protect the user from completing a bad loop. Either it should not appear in the list at all, or it should be present, but grey/unselectable.