SO is using multiple ways for user to get logged in, like using emails addresses from gmail, yahoo, facebook, stackexchange and open ID.
User might can use e.g; gmail and enjoy SO for few days and then due to any or some reason (I assume, he forgot actual way), he starts using facebook. Now SO will have multiple records for same user. Is there any way, sites can recognize there old members ?
Its just a confusion, I am curious about the concept. Like one way to register and login would be enough and would help to keep member's SINGLE record for the life. I would prefer site's own registration and login forms.