Hmm.. I personally haven't experienced this, but it could be caused when (if) you close out the FH forum without logging out of your account. I don't do this, myself, but it would make sense.
The first time is kind of a "refresh," where the leftover info for last session is overwritten for the new login,
and the second time to actually log you in.
That's my guess, anyway, since the login can sometimes be a little on the quirky side. It is a good question, though.