We do this all the time. We have a 11 lot infill project in the north end going on now along with a six unit condo complex. We've done well with this strategy and have been able to generate IRRs in the 30%+ range consistently.
There is a process with this strategy that you just need to go along with. You can't and won't change it so you just need to budget that time into your deal. The 11 lot sub will take us about a year total from the day we applied to subdivide to the day we sell our last lot. Our condo complex will be about 6 months worth of administrative and design work before we can start construction.
The best advice I can offer on this is to pre-flight your deal with the appropriate folks at the city before you start applying for permits. Just about everyone will be helpful at the city and will candidly tell you your chances of getting this deal or that deal done though the system. From a personality perspective, it's best to be a bit laid back when talking to the city folks. While they are helpful, they don't care about the economics of your project. Know that they can make your life easy or very hard. If it's the latter, there is little recourse so be nice. I've seen too many developers or wanna-be developers pound their fists on the desk only to walk out with their tails between their legs.
From a marketing perspective, we knock doors. Nothing else really works because you'll need time to get your deal done. Very few seller's agents will wait 6 months for a commission while you go through the entitlement and design process.
Your duplex idea is a zoning issue. You should get the parcel zoned appropriately before you do anything.
Good luck.