Double authentication

Hi,
i have NoCatAuth (gateway + auth service) installed on a linux box
(Fedora Core 2). It is configured in Captive mode. Account database and
administration interface is managed via a modified version of icam.
 
Everything seemed to work fine, until i noticed that two different
clients, associated with the same account, can successfully log in at
the same time. This sounds very bad because anyone can surf under false
identity, if he can steal a valid username and password.
 
Is that a bug in NoCat or a misconfiguration?
Is there a fast way to associate an account with a mac that is supposed
to be used to surf?
 
Thanks
Fabio

-- 
No virus found in this outgoing message.
Checked by AVG Anti-Virus.
Version: 7.0.344 / Virus Database: 267.11.1/104 - Release Date:
16/09/2005