Remove the problem. Some ways to do this:
Stop the transfer of rares, like turning rares into relics or giving them the same probability as pillars. I recommend this.
Alternatively, block IP addresses with multiple connections to the game, like multiple tabs or browsers connecting to the server- that would limit testing in fun PvP duels, create lag-based bans, and end the use of more than one elements tab- say, spectator mode or trainer. Not good.
Limit people to 1 account. I'm not really sure how this would be feasibly implemented.