Login One! Premium Edition for Joomla! 3
For personal, one-man business and non-profit websites only. Limited support.
Current version 3.6.11. Compatible with Joomla! 3.5 up to 3.10.11 incl. Compatible with PHP 5.3.10+ and PHP 7, PHP 8 and with MySQL 8.
Updates through the Joomla! update system.
Purchase includes one (1) year free updates.

For more information see here.
Demo: see here or our Youtube Channel.

Also available as a Business Edition.

An authentication plug-in that prevents multiple log-ins by the same user credentials, for both frontend and backend. A useful tool for websites that share or exchange potentially sensitive information amongst its registered users. Supports Two-Factor Authentication.
User needs to either sign off the first session, or wait until the first session has expired.
Super Users can always have multiple log-ins.

With this premium edition, specified user groups can be allowed multiple log-ins. The ultimate experience for professional websites however is delivered only by the Business Edition.
Install or upgrade via Joomla! backend.

System requirements:

• Joomla! 3.x only. Joomla! 3.5 or later;
• PHP 5.3.10 or higher or PHP 7, PHP 8;
• Joomla! 3.8.3 or earlier only: Session Handler (global config, tab system) set to "database" (i.e. not “PHP” and not "none");
• Standard Joomla! log-in form enabled and in use as the log-in gate;
• Browser has cookies enabled.

Basic setup instructions are here. These are also in the documentation that comes with the download package.
Please note that the plug-in must now be installed as a package. The package will install three separate plug-ins.
Configuration overview: see here.

Supported languages: Front-end: configurable. Back-end: English.

IMPORTANT NOTE WHEN USING OTHER 3RD PARTY PLUG-INS AND OTHER EXTENSIONS
Authentication and user plug-ins can be critical. The use of additional 3rd party (i.e. additional to a standard Joomla! install) authentication and user plug-ins, templates or other extensions may turn the Login One! software partly or totally ineffective, meaning that it will not - or only partly - perform the tasks for which it has been designed. 3rd Party templates may not display the plug-in notifications properly. In order to avoid disappointment, we recommend that you always test the Freeware Edition before purchasing a commercial edition.
The Premium Edition does not support user log-in by email address.

License: For personal, one-man business and non-profit use only. License terms are here.
Support: Limited support. Support terms are here. You may find the answer to your question on our forum.

Trouble Shooting guide: see here.
Refund and discount policy: see here.

You may want to try the free standard edition first. The standard edition is very basic and not configurable.