A mod is a stream moderator. I recently set up a stream that I have not made public yet. Moderators can approve or disapprove a submission, mark it safe for work or NSFW, change certain text, remove a meme that was already featured, and a number of other things.
(They would remove something already featured if it draws to much disputes or if someone points out something the memer and mod didn't realize up front.)
Also, when reposting, standard practice is to save the meme on your PC, then create new image and upload that image, re-enter the text, and then hide it by shrinking the size (Arial, 1 pt, no shadow) of the text and the box and changing the font color so it blends with the background (usually black or white), then generate the meme.
The moderator can see all those, which makes it obvious it's a repost.
Also, there's something called "steganography", which is hiding information in an image, such as having some instructions in a "painting" hanging on a wall in the background of a portrait. No one will pay attention to it except the recipient who knows to look for it, then greatly expand the image to read it. Another way to hide something is to us a VERY SIMILAR color for text. Then the recipient uses image-editing software to change the specific color, e.g., change dark red on brown to yellow in order to be able to read it. Although realistically Imgflip isn't worried about that, it IS something a mod could see.