Vesuvian Doppelganger

Undying (When this creature dies, if it had no +1/+1 counters on it, return it to the battlefield under its owner's control with a +1/+1 counter on it.)
At the beginning of each upkeep, target creature you don't control gains undying until end of turn.
{1}: If Vesuvian Doppelganger has a +1/+1 counter on it, it becomes a copy of another target creature with a +1/+1 counter on it until end of turn.