Chapter 3 Authentication for FortiOS 5.0 : Single Sign-On to Windows AD : Troubleshooting FSSO : Users on a particular computer (IP address) can not access the network : Solutions
  
Solutions
Ensure each computer has separate IP addresses.
Encourage users to logout on one machine before logging onto another machine.
If multiple users have the same username, change the usernames to be unique.
Shorten timeout timer to flush inactive sessions after a shorter time.