Unique event counting

I have a counter that counts each time a special event in a game occurs.

The code looks like this at the end of several movieClips’ last frame:

_level0.count++;
stop();

The problem is sometimes events get repeated, and though I don’t mind if the event happens again, I don’t want the repetition to count as a new number. I want it to count once and only once.

How could I code this?