I, too, have experienced this. However, after making a few careful observations, I've managed to avoid it. You can, too.
Can I assume you have the Fungi Forums bookmarked? If you come to the forums via the main page, you will be unable to retain your current log in, and will have to go through process which requires you to log in twice.
However, if you do have the forum bookmarked, it's likely that you have:
http://www.themushroomkingdom.net/board listed as the URL. When you use this URL, you have to relog in every session. However, if you change your bookmark to:
http://themushroomkingdom.net/board it will retain your login.
So, from now on, use
http://themushroomkingdom.net/board to reach this site, and you won't have any trouble.