If you get the same user listed in the chat users list multiple times, it means that you have the same user logged in into Joomla! with different sessions. The chat application works based on Joomla! sessions, so if you are logged in and if you have active sessions with the same account on different browser or different devices the same user will be listed multiple times in the Joomla! administration and even in the chat users list.
Although there is no way to avoid this completely you can minimize this issue:
- Ensure to not login multiple times with the same account with different browser/device
- Choose a low value for the Joomla! session lifetime in the global configuration
- Keep the JChat parameter 'Max inactivity time to strip out users' to a low value, max 30 seconds to strip out inactive users as quick as possible