Since all photos now have SHA-2 hashes stored with them, you should no longer be able to upload a photo to an entry if it's already there. This is only within the scope of entries; you can upload a photo twice if you don't specify an entry, but I'm not sure why you would do that anyway.
Right now I'm working on making a page that lists all the duplicate photos for the admins so we can start getting rid of them.
The MLP entry has 424 duplicates, with 22 of them being posted 3 times each.