"Glue" accounts when logging in via Vkontakte and Facebook

Question

Facebook

The situation is as follows, I am making a voting site where authentication is possible both with Facebook and through Vkontakte.
I would like if a person has both, but, for example, at work he uses Facebook, and at home he uses both, so that he has one account on the site.
How to send a request in the "background" and authenticate automatically - doper.

Is it even worth making an "exit" button?
Leave the "Vkontakte" button when logging in with Facebook? Suggest "Do you also want to log in with your VK account Ivan Ivanov"? Offer to merge accounts?

It is clear that there is most likely no single recipe, but I wonder how you did / would you do?

Answer 1

when the person is authorized, make the “other accounts” item in the settings and let him link there ... well, actually store them like this, let's say id_in_your_system | social_name | id_social
I don’t know how it works inside, but outwardly the same system at the film base, for example