Refuse SSO account creation with duplicate email
Refuse SSO account creation with duplicate email

If there is already a user with that email address, don't create a new
user during SSO.

Don't SSO into the existing account, either, because the user hasn't
previously authorized us to.

Debian has seen several users end up with duplicate accounts like this,
because they signed up through the password-based flow, and then SSOed
later.

CTPUG/wafer,CTPUG/wafer,CTPUG/wafer,CTPUG/wafer