What criteria does Apple use to determine whether a vCard sent for example as an email attachment has a duplicate? I would think there has to be more in common between the duplicates than just the given name and the family name? Does Apple say whether the given name, family name, and address enough similarities to consider two contacts duplicates of each other?
Determining whether a vCard has a duplicate on a current iOS device?
You should test by yourself: create an entry in Contacts. Then send email with different vcard with same name and first name, but variation of other fields: email for instance. And see what happens.
Note that you can create a full duplicate in Address book.