In NYC a duplex is an apartment, or sometimes part but not all of a house, that is on two-floors. For the most part, only rich people have apartments with their own inside stairway (or elevator.)
If "Auntie Mame" lived in an apartment, it's shown in the movie as a duplex. She was based on a real person
I almost rented a triplex, top 3-floors of a 5-story much smaller than average brownstone, but I thought I'd be running up and down stairs even more than when I lived in a 2-story house.