No need to send confirmation emails. Let him login with any email address. If the authentication is successful, you let him in. At this stage, it is a new accou

No need to send confirmation emails. Let him login with any email address. If the authentication is successful, you let him in. At this stage, it is a new account so give him an option to link this new account with his existing account with your service. If he had already provided all his email addresses with the first account, there is no need to authenticate one more time. Otherwise, simply authenticate once more with the original email and link the accounts if successful.