Currently a Dictionary can be merged infinite times into an already-existing Dictionary.

Note that this applies to both the GlobalResourcesDictionary, as well as merging custom ResourceDictionaries.

See attached screenshot.

Repro Steps:

1) Create a Studio Project.
2) Create a ResourceDictionary from the Assets Panel.
3) From the Resources Panel, merge it into GlobalResources.xaml
4) Repeat Step 3) as many times as desired.

Observe how the same dictionary is merged into GlobalResources as many times as the action was performed.

Expected Results: A dictionary can only be merged into another once. Ideally, we don't show, or grey-out the already-merged Dictionary in the Merge Dictionary Context Menu