How do I Collide with only 1 object

So I'm making a simulation and in it when CO2 hits a tree it becomes O2 and when O2 hits a rotting log it becomes CO2. Right now I have it set so that on a collision that gas is destroyed and a new one is spawned but the problem is that it 2 trees or logs are close together extra gas is formed. Is there anyway to only allow one instance to be made?

Without knowing how you have your events set up, it would be hard to answer..

On collision should only trigger once..

I'm guessing you have conditions to your on collision event that should be made untrue in the actions attached..

only if two instances of the object colliding with are exactly over eachother could there be an issue..

This could be handled with a pick top instance, maybe..

Well I have

CO2 on collision with tree: CO2 destroy

CO2 spawn Oxygen

trigger once on co2 spawn?

Didn't even know that was a thing thanks! Hopefully it'll work!