An email address can only belong to one contact. When updating a contact you'll be prompted to either Move or Merge the two.
Moving a Contact: Move one of the email addresses from one contact to another, while keeping both contacts in place.
Merging a Contact: Merge two contacts into one with all of the email addresses under one updated contact name.
*When merged, contacts emails will remain assigned to the groups they originally were, using the most recent contact name.