Say you have duplicates in your database (multiple contacts with the same email), and you wish to update them all.

In order to uniquely identify and update those contacts, you would need their RecipientID which is unique.

Currently only the SelectRecipientData endpoint can return RecipientID, but if there are duplicates in your database, only the oldest record's RecipientID will be returned.

If you needed to retrieve ALL contacts that match your search criteria, you can use ContactByLookup as that can return multiple contacts.

It returns all contact fields (even the custom ones) for all matching contacts, but the RecipientID...

Adding RecipientID to ContactByLookup would be very beneficial to uniquely identify the contacts, and could avoid adding an another custom surrogate key to your database, as a workaround.