If this is your case you may want to consider Mozilla Persona, which will default to OAuth for some providers such as GMail.
We do have bizarre situations like people logging in using Twitter or Yahoo accounts bound to GMail, but they are rarely an issue. It's more frequent the case where a user is trying to open their account page using the wrong GMail account.
Anyway, our plan is to detect multiple accounts with the same e-mail address and offer the user to merge them, but we are not sure about how to do this since e-mail addresses are not immutable.
We do have bizarre situations like people logging in using Twitter or Yahoo accounts bound to GMail, but they are rarely an issue. It's more frequent the case where a user is trying to open their account page using the wrong GMail account.
Anyway, our plan is to detect multiple accounts with the same e-mail address and offer the user to merge them, but we are not sure about how to do this since e-mail addresses are not immutable.