only one user logged per session/time

[nbari at unixmexico.com]

Hello, I have a server with ssh access to the users, but how can I allow
only one user to be logged at a time.

I dont want to allow a user to login multiple times, I just want the user
to login onece and if he try to open a second coneection, refuse the user
until the session is over or the user is allready loged.


thanks in advance.