DEUF Wiki & Guide
wiki.9minecraft.net/deuf-9minecraft-mod/
What a duplicate entity UUID is, how it breaks a world, what this mod does about it, and how to switch on the repair log.
Overview
Every entity in a Minecraft world — every cow, boat, item frame, dropped item and armour stand — carries a UUID, a long identifier that is supposed to be unique for the whole save. The game relies on that: when it adds an entity whose UUID is already in use, it refuses the newcomer outright.
DEUF sits at exactly that moment. Instead of letting the game reject the entity, it hands the newcomer a freshly generated UUID and lets it in. Both entities then exist side by side and behave normally.
The mod adds no items, blocks, recipes or screens; it has a single config option and otherwise never announces itself.