Multiple emails per contact

We have a problem, that some users register in our platform with one particular email, but later they can write to us (by mail) with another email (different than the used for the registration). That is a problem for us, because you end up with a lot of "anonymous contacts" that count towards "contact limits" and moreover you , we should be able to merge a contact with another one so that any contact could have more than 1 email associated.