After this the zombies just stand on top of the player without doing any damage, is there a way to start a loop?
Try using "is overlapping" and "every 0.35 seconds" instead of "on collision".
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https://s12.postimg.org/m6wrzcxml/Screenshot_8.jpg
I used that in one my older games, worked really good, and if like 2 enemies are overlapping the damage is doubled.