weird behaviour with sessions

Hi. I'm experiencing strange behaviour with my sessions: on page A there is a logon form (UID/PWD) with target of the submit being '_blank'. So when I submit, a new window B opens, I do session_start(), check the credentials and on success relocate to the main application while passing on the session ID. The application has all sorts of links in a navigation bar, all of which now have the session ID appended. The 'logout'-link points to a script which destroys the session and closes the window (B). Now (please bear with me), when I switch back to the logon form in window A (still open) and just submit again, the process starts over BUT WITH THE SAME session ID. How can this be ?? Any idea ? Harald