Vesuvan Doppelganger’s triggered ability may cause it to become a copy of itself. A Vesuvan Doppelganger that becomes a copy of a Vesuvan Doppelganger (either itself or a different one) will gain another instance of its triggered ability. Each instance of that ability will trigger during its controller’s upkeep. They’ll all be put on the stack and resolve one at a time. The last one to resolve determines the Doppelganger’s characteristics for the rest of the turn.