New user membership messages sometimes sending duplicates

If a prospective users email is used in multiple contacts (such as SIGNING and MARKETING), then multiple messages are sent out. We should be sending one message only as its confusing and will lead to us being seen as spam.

@zfazli do we care about saying what role the user has been set in in the email? If we don't need to collect the roles, it gets way easier to maintain this and make sure we only send one email message. We can also just make it a list of roles, its just more complicated which I'm looking to avoid if its not necessary.