Trigger single collision event

For my game, I have multiple game objects for all of those I attached single collision detection script. When object collides with other object two OnCollisionEnter method executed because each object contains same script. I want single collision method execution per collision so what changes I have to do in my scripting? Basically my goal is to execute single time my code withing OnCollisionEnter method. Although this question seems to be easy but frustrating me from long time.