New user.id being generated while extending the session object. Is this normal?

[YO-SC]

Hello devs, I am following the official documentation on how to extend the session object. I noticed during development that every time I signIn a new user.id is being generated (even when signing in using the same account; in this case a GitHub account). Is this normal AuthJS behavior? Thanks.

auth.ts

import NextAuth from 'next-auth'
import github from 'next-auth/providers/github'

export const { handlers, signIn, signOut, auth } = NextAuth({
  providers: [github],
  callbacks: {
    jwt({ token, user }) {
      if (user) {
        // User is available during sign-in
        token.id = user.id // new user id being generated upon sign in, idk why
      }

      return token
    },
    session({ session, token }) {
      session.user.id = token.id as string

      return session
    },
  },
})

sign-in.tsx

import { signIn } from '@/auth'
import { Github } from 'lucide-react'
import { Button } from './ui/button'

export default function SignIn() {
  async function signInWithGithub() {
    'use server'

    await signIn('github')
  }

  return (
    <form action={signInWithGithub}>
      <Button className="flex justify-center gap-2" type="submit">
        <Github /> Sign in with GitHub
      </Button>
    </form>
  )
}

sample output

{
  user: {
    name: 'John Doe ',
    email: '[email protected]',
    image: 'image-url-from-github',
    id: '8ea0be81-50df-49d9-88d4-3261fe7bba7d'
  },
  expires: '2024-10-02T16:55:21.378Z'
}

then, upon a sign out and sign in with the same account:

{
  user: {
    name: 'John Doe ',
    email: '[email protected]',
    image: 'image-url-from-github',
    id: 'f47ac10b-58cc-4372-a567-0e02b2c3d479'
  },
  expires: '2024-10-02T16:55:21.378Z'
}