Research Currently Listed Active Properties
Get in your car and go check them out…
If current active listings compare similarly to your multi-unit property, drive by and check out the property’s curb appeal. It would be nice to see the interior but since the units are probably occupied, you will have to rely on MLS photos. It's likely that there may not be any recent photos unless the property has been updated in preparation of putting it on the market.
These active properties are your competition. Look at your property and your competition through a buyer’s eyes. Why or why wouldn’t a buyer prefer your duplex over your competition. Determine your initial asking price accordingly.