Note: This doesn't work in vanilla, I've sat here for 30 minutes now trying to get it work in vanilla and it doesn't.

You can throw zombies swords, and then kill it with looting and it dupes the item you threw to the zombie.

How to reproduce on spigot:
Get a zombie
Throw a sword at it (enchanted or not)
Kill it with Looting 3
You get your sword back (and an extra)