None of the above. I don't have a section that says "SIgn in and registration page setup".
Under "General Cart Setup", I have "Add to cart requires login", but that doesn't work always. I still have users saying that they are getting the "User already exists" error on checkout, until they log in.
I'm not sure what's going on... the only reason that that particular message should pop up is if a session variable is not set.