What the mod actually does in DEUF

When an entity is added to a world, DEUF checks whether its UUID is already taken:

1. Players are skipped — their identifiers must never change.
2. If the UUID is free, nothing happens at all.
3. If it is taken, a new UUID is rolled (and re-rolled until it is genuinely unused), assigned to the newcomer, and the entity is added normally.

The repaired entity keeps everything else — its position, health, inventory, name and behaviour. Only the internal identifier changes, and only for the entity that arrived second.

Without the mod that second entity would simply not be added: it disappears, and the log records a duplicate-UUID warning in its place.