Can you buy or sell one half of a duplex

[Karen Russell]

Hi out there.  Not sure if this is a dumb question or not, but I'm looking at a duplex that is for sale (house no. 4764-4766) and when I'm researching I find an old listing for just the top unit (describing only the top unit at 4764) which sold for $50K and then a listing for the bottom unit, describing the whole duplex but the address at 4766 and that sold for $50K.

What the???? You have to buy or sell the whole duplex, don't you???? - So the total sale price was $50K for both the units??

Thanks for clarifying.