.dupe
Type	Command
Category	none
In-game description	“Duplicates items using a book & quill.”
Default keybind	none
Source code	net/wurstclient/commands/DupeCmd.java
Supported Minecraft versions	1.16.3 - 1.17

.dupe is a chat command that allows the user to duplicate items in survival mode. It does this by corrupting the NBT data of a book and quill item so that the server fails to save the player's inventory.

The glitch behind .dupe was originally discovered by Gaider10 and published on January 22, 2021¹⁾.

Before using this command, the player must acquire a book and quill and a chest (or some other container).

1. Equip the book and quill item in the main hand slot.
2. Store the items to duplicate somewhere in the player's inventory.
3. Disconnect from the server and reconnect. This causes the server to save the player's inventory.
4. Take the items to duplicate out of the player's inventory (e.g. by storing them in a chest).
5. Run the .dupe command. The player should be immediately kicked from the server, but this time the inventory is not saved.
6. After reconnecting to the server, the items will still be in the player's inventory, but the same items are now also in the chest.

This process can be repeated over and over again. The book and quill can just be left in the main hand slot, but the disconnecting and reconnecting step has to be done every time.

Pressing F3 shows the type of server in the top-left corner.

This item duplication glitch is known to work on vanilla Minecraft 1.16.3 - 1.17 servers. Bukkit-based servers seem to have patched this glitch.

When this glitch doesn't work on a server, the player will not be kicked and will instead receive a book titled “If you can see this, it didn't work”. When it doesn't work on Paper servers, the player is kicked with the message “Book too large!”. When it doesn't work in Minecraft 1.17.1-pre1 and later versions, the player gets kicked as usual, but the items don't get duplicated.