AspNet Core External Authentication with Both Google and Facebook
<p>I am trying to implement the Form-Authentication in ASP.Net Core with Both Google and Facebook Authentications.  I followed some tutorials and after some struggles, I managed to make it work both.</p>

<p>However, the problem is that I cannot use both authentications for the same email.  </p>

<p>For example, my email is 'ttcg@gmail.com'.
I used Facebook authentication to log in first... Registered my email and it worked successfully and put my record into 'dbo.ASPNetUsers' table.</p>

<p>Then I logged out, clicked on Google Authentication to log in.  It authenticated successfully, but when I tried to register it keeps saying that my email is already taken.</p>

<p><a href="https://i.stack.imgur.com/2VjJg.png" rel="nofollow"><img src="https://i.stack.imgur.com/2VjJg.png" alt="enter image description here"></a></p>

<p>I tried to do the same thing for other online websites (Eg, Stackoverflow).  I used the same email for both Google and Facebook and the website knows, I am the same person and both my login / claims are linked even though they come from different places (Google &amp; Facebook).</p>

<p><a href="https://i.stack.imgur.com/ansGw.png" rel="nofollow"><img src="https://i.stack.imgur.com/ansGw.png" alt="enter image description here"></a></p>

<p>I would like to have that feature in my website and could you please let me know how could I achieve that.  </p>

<p>In theory, it should put another line in 'dbo.AspNetUserLogins' and should link the same UserId with multiple logins.  </p>

<p>Do I need to implement my own SignInManager.SignInAsync method to achieve that feature?  Or am I missing any configuration?</p>

<p><a href="https://i.stack.imgur.com/hSsFo.png" rel="nofollow"><img src="https://i.stack.imgur.com/hSsFo.png" alt="enter image description here"></a></p>