The Donkey Dupe is one of the only dupes from 2018 to have been patched then unpatched then repatched
How it worked
You need wwe or any client that has a .vanish or dismount command. You would take a donkey or llama (Could be called the llama/donkey dupe) put a chest on it and hop on it. When on you do .vanish dismount and when done you would be "off" the donkey but you were still on it after making a 150 long block tunnel you would walk down it while waiting for chunks to load. When at the end you .vanish remount it will teleport you back and create a second donkey and which you could take the items out of and you would have the same items x2
As said before its been officially patched as of April 2, 2020 with a message saying Chest on donkeys,llamas, and mules have been disabled.