Probably not. Duplexes are attached and usually have one owner.

https://en.wikipedia.org/wiki/Duplex_(building)

They were popular in many cities in the 70s as starter homes. The idea was that you'd buy the whole thing, then rent out one half to help offset your mortgage.

But SB9 also allows lot-splitting if the plot is large enough, right? Also allows duplexes original lot could be x2 and each of those can have one duplex.

Obviously this lot-splitting may not work in areas like Los Altos where each single family lot by city law is required to be 10k sqft or larger (unsure how local laws will interact with SB9).

But question @koolba asks is still valid - can you recursively split large lots into many smaller lots?