I see that we misunderstood each other. I was talking about a building mapped with one outline and tagged as “duplex”. In this case one cannot know which type of “duplex” this is, if there is no subtag explaining it. The only thing we know it is a 2-family-home, and that’s as good as tagging the same building as “house”.
If it is a side-by-side duplex it is surely better to tag it with 2 outlines (and that is what I am doing) but in this case I do not need a special tag for it. I can tag both as “house” and by the geometry it is already clear that this is a semidetached housing unit.
That is why I believe “duplex” as well as “semi-detached” (and also “detached” btw., not to talk about “maisonette”) are quite useless tags just adding complexity and confusion instead of making tagging better. Using “house” for any residential building incorporating 1-2 units covers it all and does not offer much space for tagging mistakes, even if lateron the building is redesigned, splitting a 1-family-home into 2 apartments or vice versa, or even splitting off a third small unit in the roof or the basement which all happens very often with this type of house.
Anyhow, if you want to expressively tag a house as a building with 2 units, you still have the option to go for
Off topic:
You have expressed that earlier, but this is not the issue here 
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