In terms of what you should do with your profits/additional savings, there is a pure math answer and an emotional answer.The pure math answer is to do whatever makes/saves you the most money. 

Take the emotion out of that side of the transaction.

As others have said, duplexes and even 4 unit buildings are often sold on emotion and not the financial performance.

You will, however, need to be able to print/export your financials by property (Customer/Class/Location -- whichever you choose) and be disciplined about entering that information into every transaction. 

Negotiated an Owner Finance and closed escrow after 2 weeks.Price: $72,500Zillow Est: $111,000Summary:It was an absolute emotional roller coaster, navigating through all the small details that wont be a big deal next time around.

My impression is that those who are against it are basing it on an emotional response.

You sound too emotional to be investing in real estate.

Repeat.What was emotional at first when I first achieved financial freedom is now strictly mechanical as I expand and strengthen my financial freedom going forward.

I am a business man and like to look for solutions rather than emotionally react." 

Everybody here seems to hate appraisers, but most of the time they are right about the value of a house, the problem is that sellers don't want to sell a house for what it's true value is and many buyers are willing to pay more than what a house's true value is because they are impatient, or make an emotional decision, or have a realtor who is a good salesperson etc.