Single family home will have a better resale value if put in the same locality as the duplex.
However, a lesser price duplex might attract more buyers (lower price range = more buyers).
Single family home has no neighbors - and privacy is appreciated by buyers.
It has a back/side/front yard - with no one telling you to match your flowers to the siding color, and
kids/dogs can run everywhere.
There is no "discussion" of when to change the roof or siding, or porch (if shared).
Your neighbor won't start defaulting on his mortgage - putting you in an unpleasant situation.
Sudden low resale of your neighbor won't decrease your property value.
Hope this helps,
Beachfront Realty, Inc.