Mail changing address book

I save all my contacts in Address Book in the First Name field (not using Last Name Field). When I use Mail to send emails, somehow last name of the sender/recipient gets added to that person’s contact. So John Harris becomes John Harris Harris – super annoying!
Someone suggested to delete all from Recent Recipients in Mail – I did that, it helped but only in relation to those contacts who have been deleted from Recent Recipients – is there a way to disable Mail poaching surnames from my emails and inserting them into address book automatically?
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Jairo