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UUID Conflict

A player cannot join because their UUID is already considered in use. Two identities map to the same UUID, common on offline-mode servers or after switching between online and offline modes.

What does this error mean?
Every player is keyed by a UUID. 'UUID already in use' means the server thinks that UUID is connected or registered to a different identity, often because offline-mode generates UUIDs from usernames, which can collide or differ from online UUIDs. 
UUID already in use
Most Common Causes
  • Switching between online and offline mode (UUIDs differ between them).
  • Offline-mode UUID collisions from identical usernames.
  • A ghost session still holding the UUID.
  • Mixed authentication behind a proxy (BungeeCord/Velocity).
  • Manually edited data files with conflicting UUIDs.
How To Diagnose
  1. Check whether the server is online or offline mode.
  2. If behind a proxy, confirm IP forwarding/auth is set up correctly.
  3. Look for a lingering session for that UUID.
  4. Inspect usercache.json and playerdata for duplicates.
Recommended Fixes
  • Use consistent auth mode
    Stick to one mode, switching online/offline changes everyone's UUID and breaks data.
  • Configure proxy forwarding
    On BungeeCord/Velocity, set up proper IP forwarding so UUIDs stay consistent.
  • Clear a ghost session
    Restart the server to drop any stuck session holding the UUID.
  • Resolve data conflicts
    Fix or remove duplicate UUID entries in player data files.
Analyze My Log
Frequently Asked Questions

Offline mode derives the UUID from the username, so identity depends entirely on the name.

Very likely, online and offline produce different UUIDs, so player data no longer matches.

Yes, without correct forwarding, backend servers can misassign UUIDs.