Minecraft wiki entry for Block_and_item_duplication, associated text: Working in 1.15.2 and in 1.16 snapshots up to 20w12a (Patched in 20w16a and in paper). You need a tamed donkey or llama equipped with a chest. If a player shift-clicks the donkey / llama, that player may view the donkey's inventory and place items. If a second player rides the donkey, the first player will continue to view the donkey's /llama's inventory, even though it is not normally possible to view a donkey's inventory when it is already being ridden by a player. If the second player disconnects, the donkey will disappear along with them, and the donkey's inventory will be considered by the game as part of that player's data. However, the game will keep a copy of the donkey's inventory and GUI temporarily loaded for the first player, who can then reclaim the items. When the second player reconnects, the donkey's inventory will contain what was originally in it before the second player disconnected. The items can once again be taken from the donkey's inventory, resulting in x2 duplication.