The current trend for duplicates seems to be:
-they get reported in a comment by an observant user.
-nobody does anything about it.
I noticed, however, that post #146991 was a parent of a duplicate of itself, which gave me an idea. A modification of the child system would work rather well for noting duplicates; you make the 'best' (least artifacts, highest res) version of the image the 'parent' of all other versions, and the duplicate-parent would have a colored border or be in some list of posts with duplicates or similar. A moderator could then delete the lesser duplicates, perhaps using some kind of system to merge favorites and tags, and give the remaining copy the highest score of all existing ones.
Updated by Almatia