[User Registration] Prevent the creation of different login accounts for a same email address

Steps to reproduce:

1. Invite a user: provide an email address and a username.
2. With Administrator, approve the invitation.
3. Validate the invitation by clicking on the invitation email link.
4. With Administrator, do another invitation: provide the same email address as for the 1st invitation, but choose another username.
5. With Administrator, approve the invitation.
   -> An invitation email is sent to the email address, with the new login and a new password.

=> It would be nicer to prevent users from inviting the same user (i.e. the same email address) with different logins. (This should be taken into account for bulk invitation as well.)