What happens when a creature with undying returns to the battlefield with a +1/+1 counter and then dies again?

Undying reads: '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.' (CR 702.92a). The critical condition is no +1/+1 counters at the moment it dies.

When a creature with undying first dies (with no +1/+1 counters), the ability triggers and it returns with one +1/+1 counter. That counter remains on the creature as long as it stays on the battlefield. If it dies again, undying checks at the moment of death — the creature now has a +1/+1 counter, so the condition is not met and undying does not trigger. The creature goes to the graveyard and stays there.

However, if something removes that +1/+1 counter before the creature dies again (e.g., Power Conduit, Fate Transfer, or a spell that removes counters), the creature would once again have no +1/+1 counters, and undying would trigger on its next death. So the answer can depend on whether the counter was removed.

Example: You control Strangleroot Geist (which has undying). It dies in combat with no +1/+1 counters, so undying triggers and it returns with a +1/+1 counter. Your opponent blocks and kills it again. Since it now has a +1/+1 counter when it dies, undying's condition is not met — it goes to the graveyard permanently.