Ah! My area of expertise! I do lots of Fan stuff, live in the Fan and have sold bunch of multifamily both for and to my investor clients.

There are lots of Fan duplex comps. Why do you say there were no comps for your duplex? And unless you have some very specific reason you need to sell off market - which is possible, don't get me wrong - you will not maximize your return unless you put the property on the open market.