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. 

Features:

• Automatically stay after Core Joomla 'User - Joomla!' plug-in
• Separate front-end and back-end loginations, if 'NO' front-end session will be deleted if user login to back-end and vice versa.