MyBB Community Forums

Hey guys,
I just converted my SMF 1.17 forum to MyBB 1.2.14
Seemed ok, there were posts missing but I figured because I disabled post count in those forums.
I followed the steps to upgrade to 1.14
And I get this error at the end of the browser upgrade script:

Fatal error: [SQL] [1062] Duplicate entry 'hsfdcsffhs' for key 2
ALTER TABLE mybb_users ADD UNIQUE KEY username (username) in /home/[username]/public_html/inc/db_mysql.php on line 548

I had one plugin open. I don't know what it was, I didn't install anything. Must have been included on the default install...anyway. I couldn't access the admin cp to turn it off so I just upgraded anyway and got the above error.
Thanks.