OS X :: Sender Name Changed When Email Someone

I use gmail through my Mac Email, I have it synced up. My contacts are labeled as my friends names, for instance Joe Smith, would just be "Joe." Every time I email someone, their name automatically changes, and the last name is added, so Joe, is now Joe Smith. On the surface this doesnt seem like a terrible problem, but a lot of my friends I call by their last name, so if Joe Smith was entered under Smith, after I send an email, he is now "Smith Smith." In addition this change translates over into GMail, and my GChat contact would now be named Smith Smith or do I just have to label everyone with their first and last names?