Only Once! Premium edition

Only Once! Premium edition v1.4 for Joomla 1.5.X
For personal, one-man business and non-profit websites only. Limited support.
Version 1.4.0
For more information see here.

Also available as 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.
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 premium edition, specified user groups can be allowed multiple log-ins.
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: 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.

Discount and refund policy: see here.

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