Hold a Book and Quill in your hand and hit the book write key from the keybinds to write characters in the book.
Full Instructions:
1. Have items you want to dupe and book and quill in inventory and save them in some way (exit and rejoin easily does it).
2. Place items you want to dupe in a chest/out of your inventory.
3. Hold the book and quill and click the book write key.
4. You should be kicked and upon rejoining, items you put in chest are now also in your inventory (ie duped).
Likely patched on spigot/paper servers, but works on regular servers
