There seems to be a decent supply of duplexes on the market.

There's an under supply of housing here,  and nimby politics prevent development of housing at a rate that fixes the under supply.

Following this "Rule", you will never need any more money from other sources, since you will continuously be supplying your own...at will.Use your seed money, spend your profits...after, you've reinvested your initial profits to expand the original seed money. 

Question...what type of locks should be supplied on both sides of the door connecting the main house to the tenant house? 

If that is OK you can also ask the homeowner to finance your down payment and possibly supply the funds need to make repair or rehab.

If your lot is nearly a recently constructed Tractor Supply and a new DD going up you want to be in that are as a land owner for sure. 

I can't predict that, and I don't see any reason to think it will - but I reserve the right to be wrong.The numbers for the 1st quarter of 2018 vs 1st quarter of 2017 in Middlesex County:Homes on the market: Down 29.6%Average sale price: Up 9.7%Days on Market: Down 23.7%Months Supply: 1.2 months, Down 29.2%So there are far fewer homes on the market. 

As per RE Center of Austin, Months inventory for single-unit residential housing declined from 3.6 to 3.5 months supply, and days to sell rose from 95 to 96.

Supply and demand.

With an increasing money supply the price of goods and services also must increase, because we will have an increased amount of money fighting for the same amount of goods and services.