It’s really hard to call a duopoly much better than a monopoly. Sure you can probably identify points, but neither outcome is anywhere near efficient. I don’t know that it should necessarily be illegal. But it is market concentration.
but by definition isn't a duopoly not competing with each other? I might be wrong, but I remember it being about the two companies making deals with each other to keep both prices high and not innovate or improve, like ISPs in the US
