Only Once! Business edition

Only Once! Business edition for Joomla 1.5
For personal and business use. Support provided.
Version 1.7.1
For more information see here.

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 their registered users.
User needs to either sign off the first session, or wait until the first session has expired.
Super Administrators can always have multiple log-ins.

With this business edition, specific user groups and user IDs can be allowed or denied multiple log-ins. You can also allow/deny multiple log-ins for any ID/IP combination. The maximum number of simultaneuous log-ins for allowed users can be configured. You can also configure how to handle an attempted multiple login: display a warning message, redirect to an article, or redirect to a web page.
This edition is a 'must' for business websites, because it delivers more flexibility. See here.
Install or upgrade via Joomla! backend.

Requirements: Joomla! 1.5.15 or later. PHP 4.4.7.

Basic install, upgrade and setup instructions are in the documentation that comes with the download package.
Configuration overview: see here.

Supported languages: Frontend: configurable. Backend: English.

License: License terms are here.
Support: Extended support. Support terms are here. You may find the answer to your question on our forum.

Discount policy: see here.

You may want to try the free standard edition first. The standard edition does not include the group and user functions.