HOW TO RECREATE:

Fill a shulker, find a zombie and drop the shulker to it (or any mob with ability to pickup items), kill zombie with shulker and it'll dupe said shulker. (Can be done with any item the zombie holds)

SOLUTION

If possible, alter the code to prevent the held item being duped.
Or prevent it duping drops such as shulkers, or items with bag, backpack etc in the name as a possibly easier alteration.

**Environment Versions

Versions:**

Metallurgy:1.3.4
Forge: 14.23.5.2860
Minecraft: 1.12.2
Modpack (if available): Custom