Agreed with —
Changing the "is overlapping" event (that is tested each tick) into a trigger "on collision" that is only fired once on collision of the objects allows for the logic to be executed only once, as intended.
Are you previewing under chrome ?
If so, it's possible that you used to have so bad performance due to the use of the chrome software renderer and a bad compatibility with your hardware that made is such that there were fewer tests by second and so the code appeared to operate as intended.
In the last versions of C2, Ash implemented a check to make sure that the game is ran to the best of your hardware's capabilities on Chrome.
And so, with a greater number of tests each seconds, apparently "correct" code revealed in fact not to be the correct way to go.