So here's a silly question: does it make sense to let a person upload the same file twice?

In my mind, the answer is "no", because the data is already in their storage pool. So, rather than have the same file count twice towards their storage, the existing URL to that resource can be returned.

However, am I missing a scenario — aside from version control — where it would absolutely make sense to upload the same file more than once?

I'm thinking of photos in an album or a gallery here …