On the other hand, a cash-out refinance of your current duplex is another avenue to explore, albeit with considerations such as the risk of losing your low 3.5% interest rate and increasing your loan amount and monthly payment, thereby reducing your current positive cash flow.

This will increase the return above what can be obtained on a passive investment.The flipside of having the power to control everything is that it can be alot of work (and a full-time job if a person is putting in sweat equity).

Then, they have low ACH limits, you can ask them to increase them though. 

With insurance premiums for apartment properties increasing so dramatically in recent years, I am leaning towards trying to save some costs by increasing my deductible to the highest deductible possible.  

They are not building this, and the tenant demand continues to increase.

Also, don’t forget you will increase your rents in year 2, year 3, year 4, etc.

I try to be cordial and brief, but his issues have become increasingly bothersome.He now believes he is the victim of a serious crime, often calling to tell me the police are coming over (which they never do), or asking if I see a specific car on the street because he fears they are there to harm him.

With every unit you add the next comes faster because of increased income and financing opportunities (equity). 

I'm going to assume that their lease is either up or month-to-month and you have the legal right to raise rent now.I'd let the tenant know that they don't need to pay the increased rent and are free to move out.

Before choosing a turnkey service/company, I'd use the search feature here to find past posts like this one where members share their experiences.I am a big fan of creating and following standard operating procedures in my business.