Apple Mail duplicates first name when last name is missing

I am using Mail, Address Book and iCal synced with my Google account.

When I send an email to a contact via Apple Mail on Mac OS X and the contact does not have a lastname include, Apple Mail replaces the blank lastname with the contact's first name.

For example, if I send an email to "Dad", the contact later appears on all my synced devices as "Dad Dad".

This is only the case when I use Apple Mail on the desktop, not on iOS.

Any ideas?