Item duplication (advancement)

If you type

entitydata @e[type=minecraft:item] {PickupDelay:0}

and

advancement revoke @s only <path to this adv>

as reward commands in an advancement with the minecraft:inventory_changed-trigger and you "spam" the drop key for 30-60 sec you can duplicate the item. You will get a "real" item which you can use and keep after a rejoin.