Sellers selling 1/2 of Duplex?

[Charlie V.]

Looking around for real estate in central Maryland I'm finding some weird stuff of people posting about duplex's for sale but seemingly only focusing on one side of the property and never implying or mentioning the second side. Is this a common practice to sell half of a duplex for the price of SFHs in the area of approximate size? Or am I just reading too much into it and needing to give them a call to clarify?