I recently used an image importer that has duplicated all the images. They are attached to the right posts and the posts are fine (not duplicated).
How do I run through my media library deleting duplicate images?
Remove duplicate images from media library.
I know its too late for this answer, but can help others, you can use Media Deduper plugin to index and find duplicated image , it is also good enough to link the post to existing image.
The best way (if the duplicated images are the ones attached) is to order by date, and bulk delete all the older ones.