To be considered a duplex must it have two separate legal descriptions and have separate property tax assessments? In other words, if a house has a legal "mother in-law" with separate outside entry in the basement should it be considered a duplex or is it an accessory unit? 1004 or 1025 Form?
In my market, a duplex is usually on one lot with one legal description and one tax bill. Are there two electric meters? Are the above and below grade areas similar in size? Does zoning permit multi-family? If so, I would consider it a duplex, and report it on the 1025. If there is interior access between the two levels, I would consider it an SFR with a mother-in-law quarters in the basement. I've never had the occasion to call a setup as you've described an accessory unit.
I know appraiser are not building inspectors, however when I called the building department they told me in order to be considered a duplex they need to have separate source of heat, electrical panel, and gas meters. In this case being a basement unit it also needs to have an exterior access and have egress window.
The only condition the subject meets is an exterior entrance. We will see how the listing agent is going to react since it was marketed as a duplex. In addition, the county has not issued any building permits for the basement unit.