This is plug-in of group 'User' which prevents double login to Joomla.
How does it work? If this plug-in is installed and enabled it is checking current session of user. If session was before the new login, plug-in deletes the previous session from DB and leaves the new one. Therefore 'User - Double login' plug-in must be second after Core Joomla 'User - Joomla!' plug-in.
- Automatically stay after Core Joomla 'User - Joomla!' plug-in