I found a duplication bug, works with any item.
To reproduce:
1. Throw any item on the ground
2. Save & Exit the world
3. Come back to the world
4. Pick up the item
5. Quit by pressing Alt+F4
6. Come back to the world
Related issues
Comments
Panda4994
Mask3D_WOLF
Funny 😃 (also you spelled duplicates wrong, but no worries 😉)
migrated
This is not only dublication items but also https://mojang.atlassian.net/browse/MC-63 😉