Let’s say I have two OAuth providers, Google and GitHub with which user can create an account in my Django application. I wrote a custom user model and a custom model for storing OAuth details (provider and user ID from provider etc.).

Let’s say the user logs-in with GitHub first and creates an account. He logs out, signs-up again, but this time with Google.

Now what happens? How can I know that the user already has an account linked with GitHub? I cannot check the email because their email can be different for different platforms. I cannot use the ID of the user from OAuth provider too.

Please do not ask me to use libraries, I’m willing to learn how to implement it on my own.