When this condition is true:
Bullet -> On ovelapping with Enemy
There might be two enemies overlapping with it, and you hurt both of the in the actions.
You should pick one of those enemies, for example, add the following condition to the overlapping one:
Pick Top Enemy (It is in the Z order)
and now only the one will hirt which is on the top.
There are other picking tools also -> Pick nearest, Pick nth