It brings up this great dialog, asking:
Do you really want to overwrite this file?
And underneath it:
-- shows they have the same modtime
-- same size
-- and renders thumbnails of them both (and they're the same)
Okay, Mr. Program, maybe they're actually the same and you don't need to ask me. You obviously read the damn thing in to make the thumbnail, so you could've just hashed the file. And even if you got it from cache/exif, you know the size of it (and it's small, being a camera phone pic), so you could've read and hashed them in less time than I did clicking a confirm button.