Does undying trigger if the creature already has a +1/+1 counter when it dies?

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.' The condition 'if it had no +1/+1 counters on it' is checked as the creature leaves the battlefield (dies). If even a single +1/+1 counter was present at that moment, the ability simply does not trigger. This is covered under CR 702.93a, which defines undying, and the general triggered-ability condition rules in CR 603.4.

The check happens in the past tense — the game looks back at the creature's last known information on the battlefield to see whether it had any +1/+1 counters. If it did, undying's condition is not met and the ability never goes on the stack at all.

It does not matter how the +1/+1 counter got there. Whether it was placed by a previous undying return, by a pump spell like Increasing Savagery, or by any other effect, even one +1/+1 counter is enough to prevent undying from functioning.

Concrete example: You control a Strangleroot Geist. It dies for the first time (no counters), undying triggers, and it returns with a +1/+1 counter. Later, your opponent destroys it again. This time it has a +1/+1 counter on it when it dies, so undying does not trigger and the Geist goes to the graveyard for good.