@Gaurav Mehta Like all others have mentioned there are cheaper more efficient ways to acquire a Duplex which I agree 100%. Because you're looking for this asset to be a Build-and-Hold, here at some questions for you.
1) On the 3,700 sq ft lot, What's the actual buildable sq. ft. per the zoning? It will not be 3,700 sq ft once you satisfy the required city property setback from the boundary and the city required area to allow for underground utilities from the street.
2) Are you saying the 1,600 sq ft "two unit house" is a combined 1,600 sq ft (meaning each unit is 800 sq ft) or 1,600 sq ft each? I'm assuming 1,600 sq ft each unit. If this is the case, where is the parking for these units going to be on the 3,700 sq ft lot?
3) Are utility connections already set-up at the street? This may be the case is there was a property there previously then it was demolished, you may have to upgrade those connections depending on what was there prior, i.e. the Sewer, Gas, Electric, Water, etc.. an example would be if a SFR was there previously then was demolished the new property will require changes when building a Duplex, meaning more on your Soft Cost budget.
4) Is this a dirt lot or a concrete lot? The cost to grade it will vary depending on a concrete demo then grade, meaning you will have some engineering cost to include soil testing, Surveying cost and a list of other test before the land is construction build ready.
4) What about the Architecture cost, Are you buying one of those already designed Duplex plan kits? Those are the cheapest instead of having an architect do everything from scratch, even those will add some cost to your Soft Cost budget since the plans don't cover any of the permit cost associated with them.
6) Keep in mind that you will need 20-25% cash to secure the construction loan for this project, some banks may let you use the debt free land as a portion of the collateral on the deal to get you to that 20-25% target to secure the construction loan. This means you can't spend all of your money on purchasing the land, because you'll be left holding it vacant for a few years until you save up that 20-25% cash to secure the construction loan.
7) See if the city will let you build without an onsite parking requirement, this will only happen if there's excess street parking (you may be charged by the city for using street parking on a permanent basis) or see if there's a pay for or public parking lot nearby in which you can secure annual parking, If this is the case, see if Zoning will allow you to build Up, instead of making it a Duplex, make it a small Four unit by stacking your duplex footprint and reducing the cost to build this project.
8) Maybe look for a GC that has done many Duplexes and may have a set of plans he can use on your project, this way he can give you a better idea for the total Soft and Hard cost before you purchase the lot. The last thing you want to do is buy the lot and realize the final cost will mean you'll become profitable in year 20 on this deal.
There are a lot of other things but I won't make this post a novel, I'll just say keep in mind that since you'll be holding this property as a rental, all the money you put into the property extends your Break-Even point on recouping all of your prepaid money (land purchase, Soft and Hard construction cost, Loan cost from the construction loan) you're hoping to get back from tenants paying down your mortgage, basically when will you really start to make money on this deal? The timeframe to build in which you're asking about in the introduction isn't the only timeframe in your Cost of Capital and timeline calculation. Keep in mind the Cash Flow received once you get your Certificate of Occupancy really isn't true cash flow, it's just money received to pay yourself back that you loan the project upon acquisition.
Lastly, when looking for Lots to do Build-and-Hold projects, you want to keep in mind the more units you build the cheaper the per unit cost becomes, additionally the neighborhood Class makes a difference as well since if you're building a Duplex in a Class A or B neighborhood, your interior finishes will be much more expensive that if you're duplex is in Class C neighborhood.
Like @Frank Wong says, Why build this duplex from the ground up?