Hi there all, i'm new here and new to Xoops.
I had the double login problem described above and was about to go back to using phpnuke (which i now think is horrid). Then i searched around here and found my problem.

I tried everything listed here, reinstalled clean, emptied the session table, checked permissions...everything.

Then i found out it was my mainfile.php file.
It contained an entry refencing my site without the leading www.
Now i have altered this file, it all work perfectly.