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This feature alread exists if any file within the same "set"
is opened with the ResourceBundle editor. A way to help
prevent this is to set the default editor for *.properties
to be the ResourceBundle Editor.

Still, I agree it would be nice to prevent multiple
instances of the same files to be opened by different
editors (within Eclipse of course). I will investigate
this, but I anticipate the solution taking a while to
find... (assuming one exists).