@chokers_gossips6h, in Portugal, we understand duplex as an apartment with two floors, as described in the Learning Note. I believe the term, with this meaning, is a loanword from British English and won’t be interpreted in the same way by an American English speaker. For a US duplex, the closest term I can think of is casa geminada.
Quintal is a term we can use to refer to the outdoor area of a house in general, mainly if a (vegetable) garden can also be grown there. Otherwise, we might also just say pátio or terraço. Back yard is a suitable translation. Jardim would be translated as garden, and could be private or public. When referring to private homes, jardim would be especially (but not exclusively) suited for flower gardens/a lawn at the front.