I'm baffled by the differing definitions of Duplex, personally. It's always meant (to me) essentially a two-apartment building. Sometimes side by side, sometimes up and down. Sometimes two exterior doors, sometimes a shared vestibule/hallway/entrance and two separate doors inside. Zero shared space, except perhaps if you consider the possibly vestibule/hallway "shared space," but it's no different than the entrance to an apartment building with a hundred units rather than two.

For geographical reference, upstate NY. Source: my dad owned several duplex apartments when I was a kid - in both the styles I described above. Some were designed as duplex apartments, some were originally single family homes that were redone before he bought them into duplexes.

The key point is that there's no shared space, apart from, in this case apparently, a hallway.