User has two app-ids


I use a custom-auth function like:

 const user = await userCollection.findOne({ email: email });
  if (user) {
    if (!verifyHash(password, user.password)) {
      throw Error('Login failed');
    console.log('login succeeded', user._id.toString())
    return user._id.toString();
  } else {
    const result = await userCollection.insertOne({ email: email, password: hashPassword(password), _partition: 'pending' });
    return result.insertedId.toString();

this has been working for years.

Out of nowhere, a customer suddenly has a second new app id, how can that happen?