Often, a duplex will sell for less. Partly, it's because you're sharing a wall with another home (kind of like a townhouse). So, sometimes there's less privacy, and sometimes less yard. But it'll really vary.
In the Fairfax, Virginia, for instance, there's an area of 3/1.5 duplexes that sell, more or less, for $190,000-$230,000. You could go less than a mile away and find some single family homes, also 3/1.5 or so, same approximate age, selling for $100,000 more.
Again, that variation really depends on your locality and other factors. But you get the idea.
Hope that helps.