Items can be duplicated by placing them in a chest in the Nether and then force-closing the game

steps

1-go to nether

2-put chest

3-Put the items you want to copy in the chest

4-Go back to your world

5-close your game

6-Go back to the game and enter your world

7-go back to nether

 

You will find the items have been copied in the inventory and in the chest