Where duplicate UUIDs come from in DEUF

A healthy world does not normally produce them, because new entities get random identifiers. They appear when something copies entity data verbatim:

• a world folder or region file copied between saves, or restored from a backup on top of a running world
• structures or schematics pasted with an outside editor that kept the original entity data
• a mod that spawns entities with a hard-coded or reused UUID
• a crash or an incomplete save that leaves the same entity written into two chunks

Once two entities share an identifier, the server keeps rejecting one of them each time the chunk loads, and writes a warning every single time. In heavy cases the log fills with those warnings and the world can fail to load properly.