As far as dupes go this one is somewhat mild but still a dupe so I figured you should know about it. I think yoyos are duping armor that drops from mobs. I have a custom armor thing written for mobs in my pack and it has a long list of possible enchants along with dozens of different armor types. Considering I've gotten the same ones now and then with the exact same enchants/armor type more than should be possible I think it's safe to say yoyos are causing it (I haven't run into this before)