@swirleyDude It sounds like you really need to take a step back and learn about rental properties before buying more. Getting a grasp with business expenses, deprecation, repairs, investment mortgages, and everything else that goes along with being a landlord, particularly understanding case by case whether rental properties even make sense financially. I'm not an expert--- I do own a few and I'm still learning as I go, but understanding the basics is critical.
Paying down a mortgage isn't useful unless you plan on getting a HELOC or cash out refinance to help pay for the next property. It wraps up that cash you want access to and if for some reason its foreclosed, your extra equity in the property doesn't help you and is just more cash you'd lose. If you were asking about paying off the mortgage in one lump sum, that would be different.
I'm assuming you rent out one side of the duplex and live in the other? Mortgage interest is a business expense, but probably half, unless you're renting your side to yourself (again, not an expert). Depreciation is based on the purchase price and is a fixed annual amount regardless of your other expenses/income.
Until you have a clear plan and better understanding, it sounds like banking the money is what you should do.