Specific cards involved:
Panharmonicon
Experiment One

This didn't happen at a sanctioned event, but I'd still like to know the correct outcome in case it crops up (cause it might). Also slightly tweaking the scenario to keep it as simple as possible.

Player A controls both a Panharmonicon and an Experiment One with no +1/+1 counters on it. He then plays a Runeclaw Bear. Experiment One's Evolve ability triggers twice, as per Panharmonicon's interaction. How many +1/+1 counters does Experiment One end up with?

Based on the reminder text I ruled that it would only get one counter, and after reviewing the Comp Rules entry on Evolve I'm confident (99.9% sure) that I told true. But as I can never truly be 100% sure about my rulings, I would like someone to confirm this.

The reasoning I gave was that Evolve has an "intervening if" clause in it, so unless something gives the creature entering the battlefield +1/+1 (or more) in response to the Evolve triggers going on the stack (or before the one that would fizzle begins to resolve), then the intervening if clause causes the ability to fizzle after the Experiment gets enough counters to make its p/t the same as the creature that caused the triggers.

You are correct. You get two triggers of Evolve, but only one will add a +1/+1 counter. The Gatherer entry even explains it;



There is no functional difference in this example between having Experiment One's ability triggering twice at the same time due to Panharmonicon's ability, or having two 2/2's enter the battlefield at the same time due to Allied Reinforcements. When the first trigger resolves, it'll make Experiment One a 2/2 creature, and the second trigger resolving will do nothing since it no longer meets the requirements of the ability.