ListBox items will be added infinitely

Let me make the long story short. A part of my application is a small "Active alarm monitoring" thing which works by comparing plenty of variables by set-points and if they are critical, out of range, etc. it sorts the corresponding parameter. Whole process will operate on a timer control. Therefore, adding an item [as an alarm] keeps repeating.

I'm looking for a way of programming to do same thing "once". Thanx everyone