Is a session fixation attack / session hijack possible in JBoss and Seam?

Issue

• In certain circumstances, we were able to fixate the session for users that previously had not visited our site, shouldn't Seam be safe from even this session fixation?
• How can I prevent a session fixation attack in JBoss?
• When a user logs in the first time, a new session is created for them. If the same user login from another machine when his previous session is still valid. In this case we like to invalidate the previous session and create a new session for the user. How can we do this?