A non-performing note is unlikely to produce cash flow but profit can be harvested from enough protective equity.

Fed's "create" things by "making it rain" to create the actual producers to produce.

Real estate investments (STRs, MTRs, live in flips) have produced my best returns by a good margin.

The hedge funds and firms alone are buying more supply than is being produced. 

Same logic applies here as it does to your previous experience, there's just simply an added layer of the income produced by the property. 

Consider selling the duplex to put it into a higher income producing asset (out of state, DST, something).

When the money dries up, they'll leave just as quickly as they arrived and you may be stuck with a real mess.If it can produce more money as a short-term rental, do it yourself.

@Mark Allen Hopefully, this gold mine is still producing.

in a few short years 1 property has produced over a $1/2 million in equity, thousands in cash flow, was leverage to buy another property which does the same and leverage to buy a small cash flowing business. 

I self-manage and produce $700 in pure cash flow each month.Lessons learned?