How do I Code Lap Count in Racing Game?

hey guys, thanks for looking. Im making a racing game. How would you go about coding the finish line/lap line?

If the player or AI cross it, it raises the lap count +1 but how would I keep a player/AI from raising the lap count by simply crossing the line multiple times instead of actually racing the course?

Thanks

Checkpoints

Increment an instance checkpoint variable for each racer and only increment checkpoints as last was valid and lapCount for racer if final checkpoint was valid.

Thanks, Chrisinfinger. I think that's the solution.