Southern-ism

When we first visited in the South before we moved here, we ran into our first Southernism.
Imagine the scene. We are in a local restaurant, not a cross-country chain, but a Southern restaurant. Luke has just ordered a hamburger and French fries.
Waitress: What kin' I get you to drink?
Luke: I'd like a Coke, please."
Waitress: "What kind of coke?"
Luke, scowling—this makes no sense to him: "What?"
Waitress: "What kind of coke?"
Luke, still scowling : "A coke."
Waitress: "Oh, you want a Coke coke."
Luke, looking at the waitress like she has lost her mind: "Yes."

On the West Coast they call carbonated beverages by their individual names (Sprite, rootbeer, etc.). In the Midwest they call them “pop,” in the Northeast they call them “soda,” and in the South they call them “coke.” Still, I think it’s bizarre to say, “I’d like a 7-up coke.”