No, blatant disregard for the rules is cheating... I dont care if everyone agrees. When you disregard the rules because you want to make changes you feel are needed, and you completely discard balance and playability... you break the game.. especially in the manner mentioned above. The rules are quite clear on this particular aspect, only a single copy of a unique card per side.. as I said, the rules are in place for a reason... I'm just glad I dont have to deal with that situation in my group.
Fine then, it's cheating. Just let them "cheat" together. Why do you care what house rules (or "cheating") people do amongst themselves?