The chaotic algorithm is meant to be the hash, not the art-ification. So of course if you take two very similar hashes they will appear very similar.
Either the hashes are very different and you can already tell from them (but the art will also be different) or they are very similar (someone trying to sneak a close-enough hash by you) and then the second round of hashing leads to very different art.
If the hashes are very different, it doesn't matter that the images are similar (assuming the user indeed verifies both).
If the hashes are similar, another round of hashing before producing the image, might make the images very different.
But maybe I misunderstood?
Picking a random number and doing a keyed hash of both inputs, then comparing those, would bring out small differences in a way the attacker cannot control.