Not really a bug but a feature request mate, my customer has the same email address but different usernames, he doesn't get emails from one business though, but it's something we spoke about in the Beta period. I do agree with you though but maybe a email validation on signup.
That checks to see if that email is already in the database before you can move on. May I ask do you have multi-company addons or just one? becuase If I was you I'd merge the accounts, it's a bit hard to have customers with more than one accounts in my opinion. Had a client before having two accounts for domains and when he was looking for a certain domain, we took longer going though the accounts finding it.