privatemsg_message_recipient_changed should not be invoked if recipient to add already exists

Currently on the last line of privatemsg_message_change_recipient, hook_privatemsg_message_recipient_changed is invoked, this happens even if the recipient we're adding already exists, I don't think this is correct. This is causing multiple email notifications to be sent to the same user, if this user belongs to multiple non-user recipients (for example, roles).