No duplications no, they still happen of course, but we remove them when we find them because duplicates are pointless.
How can I report duplicates? Simply "flag for deletion"? And duplicate of id: xxxxxx? What about images that are identical, but have a different size?
Just flag for deletion on the left hand side next to the image and add reason, dupe of and add the image number.
Always report the duplicate that you want removed and not the duplicate you think should stay, some people do that xD
Also report the newer duplicate, or the smaller duplicate if one image is larger resolution, resolution overrides date, unless it's been upscaled by online software (you'll know because it becomes exactly double the resolution and sometimes looks shitty), in which case the original stays.