People should be allowed to repost. It's the reposts within a short period of time that tend to irritate users. If reposts got credit for resurrecting an image, people would just repost the current most viral images to get points.
I personally think the best action would be to advise the poster of the most recent gallery post that is the same as their image, just to make them aware of it. They can continue to submit it if they wish. Then on the image's page in the gallery, perhaps it can also list all the previous (or the most recent) submissions of that image to the gallery.
Customer support service by UserEcho