Hello. I want to realize the opportunity to enter and register on the site through social networks, but there were several questions about the implementation of this idea. There are various services to implement this method Loginza and uLogin. They seem to be comfortable, but at the same time not. Decided to implement independently. I already have a database with users, where a person is authorized on the site using an e-mail and password. The moment became interesting that if you use the services, then one person, depending on how social. networks, he is registered he can create to register more than one account, which I would not want. I tried uLogin, everything seems to be convenient, except for the above disadvantage above and even after the registration moment, the person is redirected to my site and sends him data for processing in POST. My site responds to this as an attempt to attack csrf. The same method depends on the service; in cases of malfunction, the function will not work on my site.
The first question I would like to know is: How is the registration made if I already have a user base, where is the authorization for e-mails? Can a person registered on the site previously use authorization through social. network? If you manually log in using VKontakte, it does not give the address of the mail, and if through uLogin, then there is such a possibility.
The second question I would like to know: How to prevent the possibility of cloning accounts from different social services. networks?
I was looking for such information on the Internet - I did not find it. If you find one, then the link will suffice. =)