I'd like to create such a group so that I can contact them to identify which of their multiple email addresses they want to have as their primary one.
Here's a hack that only works if you're not using the "Do not SMS" field for contacts, if you do, find a different one.
Use that field as a temporary flag to identify contacts in the group, using sql like this:
UPDATE civicrm_contact set do_not_sms = 1 where id in (SELECT contact_id FROM civicrm_email group by contact_id having count(id) > 1)
Use that field via a search to select all the multi-email contacts.
Put 'em in a group.