I only know what it was in the UK.
JoJo might remember this too, but the flats (on Avebury Avenue, JoJo) near us were a combination of duplex and ordinary apartments. The duplexes were on the top floor and just meant that they were on two floors. Ooh la la, very posh with a view of the sportsground and river,- and a price to match!:rolleyes:
Looking it up in our dictionary it says that 'duplex' means 'double' and can be applied to a house too. So, a duplex house would be semi-detached.
Yours reading-out-of-the-dictionary,
LC