Of course, one doesn't want to accidentally overwrite an image because one has forgotten that one already has an image of that name uploaded. Maybe that could be resolved by giving duplicates numbers, like "myimagename2.jpg" or whatever.

One person mentioned a few comments up about the behavior I think Flickr uses, where if you upload an image that already exists, it would take you to that image, instead of creating a copy.

Of course, one thing that could work would be to have the ability to choose which behavior you want to use. Maybe it defaults to making a copy, but you could set it to not make a copy and go to the existing image, or perhaps add an "ask me every time" option for those who might want to do a little of both?