Contacts repeating last name as first name

I have many entries in Contacts where I use only the "Last Name" field and want the "First Name" field be empty. However, Contacts (or Mail, or whomsoever, who?) is again and again adding the same name that I have in the Last Name field as first name - making i. e. "John John" out of "John". I can only prevent this by putting something (i. e. a period) into the First Name field which I started to do but don't like.

Does anybody else know this symptom and where it comes from and how to prevent it?

Why is Contacts editing my contacts on its own?