If you put 2 or more wither skeleton skulls in the dispenser and put a player or mob (for example, villager) next to it, then activate the dispenser, it will drop one wither skeleton skull on the ground AND put another one on a villager (note that only one redstone signal was given)
How to recreate:
1) Place a dispenser and put 2 or more wither skeleton skulls in it
2) Put a villager next to it or any player
3) Activate a dispenser
What should happen?
The dispenser should only put a wither skeleton skull on villager's head
The dispenser puts one wither skeleton skull on the villager and drops another one on the ground all in one redstone signal