Google Play Services Authentication Problem

I've successfully added Google play services to my game. I can sign in, display the user's name, and save data to the cloud. But there's a problem; when I press the sign in button and it asks which email address I want to sign in with, if I try to add a new account but press back before doing so, several things go wrong:

• It opens the window to ask me to choose an email address again, but regardless of which account I choose or whether I press OK or cancel, it pops up a second window afterwards anyway.

• If I select my account in the second window, the standard notification thingo appears up the top saying "signed in as username", but the callback my code receives has the success parameter as false, even though it should be true for a successful sign in.

All I'm doing is calling PlayGamesPlatform.Activate(); once and then Social.localUser.Authenticate(...); once, so I have no idea why it would pop up two windows or why it would tell me it failed even if it didn't. Any ideas?