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.
A duopoly is absolutely much better than a monopoly. You've got competition pushing you improve.
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
No, that is not a characteristic of a duopoly and in many cases is illegal.
Oh it’s illegal so they won’t do it or have a zillion ways to work around it while being technically legal.
Anticonsumer practices really need a test of the impact on consumers, similar to how hiring practices are tested. There is a give and take in most broadband suppliers, for example, where there is no written agreement to not compete but they don't compete.