So is the duplex even for sale? I'm not sure I'm following. I suggest watching this video:
@Kevin Krysty Jr. Have you tried searching for similar properties that have sold in your area on Zillow? You can typically find duplexes/triplexes by setting your search criteria to "recently sold" and "Apartments" on Zillow as that's where small multifamilies show up on that site. If you know of neighborhoods with similar duplexes you could also try looking the addresses up on your county tax assessor website to see what they last sold for. At the end of the day though, what you should pay and what people are paying are two different things, so while I'd try and get a feel for what duplexes have sold for in your area, I'd also calculate what the most you can pay and still make decent cashflow and use that number to guide any offers you make.
@Kevin Krysty Jr. Sure thing, you also might see if you can talk your landlord into seller financing, there are lots of articles on BP about the pros/cons of seller financing so make sure you read up on it.