Zombies have an extra timed hitbox layer. If you were to kill a zombie and keep swipping at the area he died the hitbox is still there for atleast 2 seconds. Every time you swipe at that area within the 2 second time the game will play the zombie dies sound.
This is very difficult to deal with when you have a zombie spawner with 10 zombies stuck in a whole and you have to pause your swings to be able to kill another zombie.