What is a duplex? Homes come in many shapes and configurations, and one type that’s popular with extended families or budding landlords is a duplex. It’s not just one home, but two. The units or apartments may be stacked one atop the other on separate floors, or they may be side by side with a shared wall. A duplex may also be called a “multifamily dwelling,” because more than one family can live in this type of building.