In almost any field that are generalists and there are specialists. If you have a heart condition then a cardiologist for example will have special insight beyond your family’s general practitioner.
In real estate, it is no different. A duplex specialist is someone who knows the basic mechanics of a real estate transaction but is also someone who understands the issues related to multifamily property and the duplex style in particular.
They will know where the duplex listings are in your local market and most likely will have some insight on the investment viability that they have.
So to define what a duplex specialist is, we would define it as a real estate professional who can explain property cash flow, and rental issues for a duplex investor and someone who can fit the needs of a home buyer to the attributes of a duplex in the case of a owner occupied duplex buyer.