(Stuff everyone already knows)
When flagging reposts that aren't exactly identical, we prefer higher resolution, and usually delete the post with smaller res, jpg versions if png already exists, and bigger file-sizes
While I'm still working on flagging the posts I've listed here near the bottom of forum #130394, I've notice something.
Quite a few times I've found reposts where the pictures are identical (same res and file format) however one file, ususally the newer one is slightly bigger, which didn't make a whole lot of sense to me. So I installed a addon to my browser to see the EXIF data and sure enough the older posts usually had no EXIF data and the new ones had empty/useless EXIF stored. Example http://imgur.com/E0GqEd1
Also I'm also pretty sure that some of the reposts are just png version of jpg that have been sent through ImageMagick or something similar, because there isn't any more detail, although that can be hard to tell.
If the source file format is different from what's been reposted vs what was already here that could be a clue, but can't always be trusted. If the jpg version has Photoshop exif data left by the artist and the png version doesn't that could also be a clue as well, if it's not very obvious.
Anyways thought I'd share this little bit of insight, that might help someone in the future with figuring out which post to delete if/when debates about it come (pfff drama never happens on furry related websites)
Updated