Do you think it is illegal to change the upper and lower floors into duplex buildings?

This is an illegal building. Because you changed the internal structure of the building, it must be illegal. Moreover, because you have changed the two floors into duplex, you have reformed the load-bearing structure in the building and changed all kinds of lines and pipes inside, which will easily affect the safety of the building, and will also affect the circuits, water pipes, gas pipes and fire pipes, and will delay the rescue in case of danger.