"... I consistently get two login screens!
I enter credentials correctly and am immediately presented with a second login screen (no errors). Authenticating against the second screen completes a normal login to the Ubuntu desktop."
The above is a quotation from a problem reported on 7. May 2012 by Soren Mogensen. See User login cycles. I have the exact same problem, and was hoping that Soren might have received an answer which would enlighten others, but unfortunately Soren's problem was reported as part of an "answer" to another problem, and the only "solution" on that occasion was advice to create a new question.
I can login, I just have to present my credentials twice, so this is NOT a duplicate of any "unable to login" problem. My concern is that this looks like a classic phishing setup, where one of the two login screens is a fake designed to harvest my ID and password for some nefarious purpose. If Ubuntu has been susceptible to phishing attacks for nearly a year now, then I imagine that would be a matter of enormous concern.