How do I make an event only trigger once?

in my scenario, I'm trying to make a platform move on collision with the character. I'm using a tween to make it move back and forth but every time the character collides with the platform it updates the tween to the same location and the same time making the platform move slower because it has already moved part of the distance but the time resets. how do I make it only activate once?

Try adding a condition "Tween is NOT playing" to "On collision" event.