How to value a duplex in area of only SFRs?

[Sam T.]

How does everyone estimate value for a duplex in a neighborhood where there are no other duplexes? I'm sure there's no exact formula here (I hope I'm wrong about that though!) but what would be a rough way to approximate based on the nearby single family houses? I'm completely lost on how to estimate retail value of a particular duplex with this situation, and would love any insight you all could share.

[Nicholas LaGatta]

I would use sales comparison approach, same as a single family.  You should be able to get in the ballpark with price per square foot and adjustments for room/bath count, amenities, locations, etc.  I might adjust down slightly for a duplex in most areas, since the number of buyers would be more limited.  
I have two duplexes and have analyzed dozens more.  The math seems to hold true to the above, so far.