This is not an error, but an intentional feature of nextauth.js. A user registers with, for example, their Discord account. The next time they login they must use Discord, and not Google, Twitter, Facebook etc. This is explained in the Next auth docs under "When I sign in with another account with the same email address, why are accounts not linked automatically?".

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This is intention to avoid security compromise. Read more here:

https://next-auth.js.org/configuration/providers/oauth#allowdangerousemailaccountlinking-option

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(NOT SECURE) Account can be forcefully linked using the flag set to true (NOT SECURE)

GoogleProvider({

    clientId: process.env.GOOGLE_CLIENT_ID || '',

    clientSecret: process.env.GOOGLE_CLIENT_SECRET || '',

    allowDangerousEmailAccountLinking: true,

}),

This errror happend when you use github account which have same email which you

already login with other method like email and password for twitter or something.