Okay, I got a website where you can register yourself and login. You can also login with your facebook, twitter or linkedin account.

It is important that users only have one account registered. So somehow, I want to merge the accounts of users if they use different methods to login. What is the best solution to solve this?

For instance, the user logs in with his Facebook account. I use the data to register an account for him automatically. Should I sent an e-mail with an username and password of our website? (If this is okay with the policy of Facebook). Should I give them a second screen where they can fill in an username and password? But that&#39;s not the idea behind logging in with your Facebook account. It should simplify your procedure to participate.

It&#39;s also possible the user has registered himself on our website and the next time he logs in with his twitter account. How can I merge these 2 accounts as one? What&#39;s the best way?

So basically my question is: I got 4 different ways a user becomes a member of our website. How can I make sure all these 4 ways only create one account if a user decides to use multiple ways? What&#39;s the best flow to make sure that it doesn&#39;t become a hassle for the user himself?


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Edit: 

3 years after I asked this question, I am giving the answer myself in a series of articles: 
https://www.peternijssen.nl/social-network-authentication-setup/    
https://www.peternijssen.nl/social-network-authentication-google/    
https://www.peternijssen.nl/social-network-authentication-merging-accounts/   
https://www.peternijssen.nl/social-network-authentication-twitter-facebook/   

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Architecture for merging multiple user accounts together