I have a bunch of wallpapers in ~/Pictures/Wallpapers. Whenever I use the "Set as Wallpaper" option in nautilus, I get a dialog asking if I want to overwrite the file with itself. If I agree, the wallpaper will get set, otherwise it won't.

While this would probably be simple to fix, I also doubt the behaviour of overwriting files in this directory if they happen to have the same name, and would rather have them automatically renamed to something else.