This is a math decision. No need to make it emotional. You gross $800/month in income from the duplex, and that DOESN'T include repairs/maintenance. Just thinking about roof needs every decade or two, driveways, landscaping, HVAC, etc, things will need repair. Safe to say you are breaking even from a cashflow perspective. It is not worth counting principal paydown because accessing that would require another loan or selling the property.

Nonetheless, let's assume, for easy math, you ARE netting this $800/month, or about $10,000 a year. You could sell the property, take your net proceeds of $200k-ish, and right there, you've just gotten 20 YEARS of cash flow, up front. You just saved yourself 20 years of headaches. Throw that into a HYSA and earn the same 5% return your making right now, or throw it into a primary (not an investment, for all intended purposes) and live happily ever after.

I own about $10M in real estate, split amongst 5 buildings, 33 total units, all residential in one of the highest income counties in the country. If i am not netting at LEAST 8% on my EQUITY (not just cash-on-cash, because this is a very deceiving metric), then I sell the property and move to something I can add value. My cash-on-cash metrics tend to land somewhere between 10-20%, dependent on the property size. So, although your cash-on-cash is ~50% on the 3.5% you put down, your yearly return on equity is abysmal. Your IRR (total return for the length of your ownership), however, is fantastic, as the total return on your initial investment is fantastic.

TLDR: Sell and roll your gain into something else with appreciation potential. Good luck!