A detailed description on the Lava Dupe, the unpatchable dupe method

Create a 3x3 hole at least 8 chunks away from your spawn point (set your spawn with a bed).

Make sure the hole is 3 blocks deep as well.

Put 9 chests on the bottom, then 9 hoppers the level above that.

Fill the last level above the hoppers with lava source blocks.

You need to build up a platform at least 16 blocks high. Take all the items you want to dupe and put it in a donkey with a chest on. Hit the donkey until it has half a heart left. Also damage yourself until you have half a heart.

You need to ride the donkey over the lava, and then vanish dismount off of it while in the air and /kill. This is easier if the platform is higher up.

When you go back to the hole and go down to the chests, you will have your items duped!

This works because of a loophole in a plugin Hause installed.

You see, someone found an exploit where you could burn items at a fast rate, which cause immense amount of lag because it sends a packet to the server saying the item was burned, and the server needs to change the player data file for your player and the level.dat file. Doing this at a large and fast rate kills the server basically.

Because of this, hause created a plugin which made it so items won’t burn if you aren’t in render distance, hence why your spawn point has to be at least 8 chunks to work (the server render distance is capped at 8 chunks.) Another thing in the plugin is the server creates a duplicate of the items set to burn to change the player data before it is burned. It only does this when the items are guaranteed to burn.

This works because the vanish dismount module makes it so the server thinks you are still on the donkey, but you /kill. It creates the duplicate of the items and the original items also go through the lava into the hoppers into the chests, and you have duplicates of the items.