Chapter 16 SSL VPN for FortiOS 5.0 : Basic Configuration : User accounts and groups : Authentication of remote users : Allow one time login per user
  
Allow one time login per user
You can set the SSL VPN tunnel such that each user can only log into the tunnel one time concurrently per user per login. That is, once logged into the portal, they cannot go to another system and log in with the same credentials again.
To do this, go to VPN > SSL > Portal and select to disable Allow Multiple Concurrent Sessions for Each User. It is enabled by default.
To configure in the CLI, enter the commands:
config vpn ssl web portal
edit <portal_name>
set limit-user-logins enable
end
Once set, once the user has logged in, no other user can use the same login credentials.
See Also
Setting the client authentication timeout
Strong authentication with security certificates