You could do it a few ways.
-Set a timer for the 'cancel'(deny) and 'accept' buttons. One player can click it, then the other play must wait 'x 'amount of time to click the corresponding button. This would eliminate any of them clicking the cancel or accept, or whatever it may be, at the same time to enable a dupe.
-Allow it so that if a player logs out, the opponent automatically wins the loot. Th losing player would then have to wait 'x' amount of time to log back in to ensure that no 'quick dupes' were enabled.
-Cancel or disable any buttons or actions that promote the dupe, whilst in the duel or duel interfaces.
-Disable the duel system
-Disable trade within 'x' amount of time after a duel.