Comment 5 for bug 692614

> Are the multiple emails causing a problem or is it just an annoyance?

As Sergio suggested on the forum, it is possible though to create two users with the same email by registering twice using different links and specifying different usernames. I see two possible solutions:

1. Leave multiple registration records in usr_registration as it is, but check at the registration key processing that the user with the same email does not exist in usr table.
2. At the registration, ensure that only one record for given email exist (e.g. each consequent registration attempt the record with the same email is being updated with new key and all the data).