That's interesting, so perhaps the list bugs because the GUID is a duplicate. Changing the file name must have some other effect that puts the custom one over the default one, since some filenames do work while others don't. Good find.
GUIDs must be globally unique in the game, as it is used for identifying objects about as often as names are.
If you copy any game asset that has its own GUID, generate a new ID for it. Doesn't matter what the new ID is, it doesn't have any requirements other than the formatting iself.
Just use this and you're set:
http://www.guidgenerator.com
