When a shulker box is broken by one player whilst another player is rapidly inserting and removing an item from it, it's possible that item may duplicate.

Reproduction

This is a very unreliable reproduction and may take up to 20 tries to successfully reproduce

1. Grab a friend in Survival mode with a pickaxe
2. Place an empty shulker box of any color on the ground
3. /give yourself a nether star or obtain one
4. /give your friend a diamond pickaxe
5. Open the shulker box, drag the nether star and then rapidly left click into the box (as if rapidly inserting and removing the star)
6. Whilst doing so, said friend should break the box with the pickaxe
7. When box is broken, place on ground and check contents

If the nether star is inside and either you or friend have a star or it is on the ground, the reproduction has succeeded. If not, repeat from step 2. It may take up to twenty tries.

Testing

This issue was originally reported on PaperSpigot by guccihub. This has since been repeatedly tested by me on PaperSpigot, Spigot and finally vanilla 1.11. I am confident this is a vanilla 1.11 server issue.

In all my testing, I used two vanilla 1.11 clients connected to the same server at the same time, across two computers.

• Original report: https://github.com/PaperMC/Paper/issues/488
• Live 1.11 vanilla - reproducible
• Live git-Paper-924 with 28 plugins - reproducible
• Live git-Paper-924 with 0 plugins - reproducible
• Live git-Paper-924 with 0 plugins and default paper.yml - reproducible
• Live CraftBukkit version git-Spigot-16c940b-ac99d7d with 0 plugins - reproducible