You may only submit this guestbook once per session!

By default GBook will only allow posting one message per user per browser session, to prevent flooding (repeatedly posting the same message).

If you get an error saying You may only submit this guestbook once per session! you will need to close all browser windows before being able to post a new message.

As a Webmaster you can disable this feature for your guestbook by setting $settings['one_per_session'] to 0;