What is a dual carriageway? Does it have more than one lane in each direction? How many? Is it fast? How fast?
It doesn’t matter what your answers were to those questions; they have nothing to do with dual carriageways. The defining factor for a dual carriageway is that the opposite directions are separated by some kind of physical barrier. The barrier could be concrete bollards, metal crash barriers, a raised kerb, or just a simple strip of grass. The number of lanes in each direction can vary from one, two, three, in fact any number. And it doesn’t need to be the same number in each direction.
So if you see another driver tootling along at 70mph on a national speed limit road with a barrier between then opposite directions, then they are not breaking the law, even if there is only one lane in each direction.