A friend of mine just asked me why he couldn't upload an image to Wikipedia.
It turned out that he was trying to replace an image that exists at the Commons.
The error message is confusing/misleading. It insists that you use a different filename. It does not mention at all the possibility of replacing the image on Commons. I think it should recommend to replace the image at the Commons, and only if that is undesired, use a different filename. Otherwise his image won't replace the old, and the old would remain at the Commons and his improvement would go unused.
Timwi