For some reason it happens quite often that mp3tag sees two identical copies of the same album art in an mp3 file. I can only speculate that someone (some software?) tries to add twice, for whatever reason ...
... or is there a different explanation to it? I'm mainly curious. Of course it would be nice to have some automated utility that cleans up this (duplicated megabyte art in each file in an album can actually take some space on a portable device ...) - but I'd be surprised if someone has written it.
And actually, double cover art - both with being e.g. "Album cover" - seems invisible in foobar2000, but that is an application-specific question that maybe should not go in an mp3 forum.
Do you have iTunes?
I have a vague memory of iTunes doing "something" to my files and it think it was something related to the artwork.
All apples in this home are edible.
But if a "big" application does this, then I guess it could affect several demo recordings that artists and small (= amateur) labels are sharing around.