A duopoly only means there are two. For example in the eastern part of the US we have QuickCheck and WAWA gas stations. If there was a small town that only had two gas stations and they were QuickCheck and WAWA it could rightfully be said there was a duopoly. That there is this "duopoly" is not necessarily bad unless the two gas stations collude on a price fixing scheme or other such illegal activities or somehow figure out how to restrict competition illegally in this small town. If this happens the pro-antitrust folks do have a point.
But imo, this does not apply to the ios vs android app store debate because of the rational I posted above, even if some don't agree with it. It seems the app store marketplace between this "duopoly" is rife with competition. Although some still call the ios app store a monopoly there hasn't been any case law validating this. Hence to side-step this issue the DMA was created.
It has been said apple violated anti-trust laws. It's true there was a finding from one of the eight or nine points in the Epic vs Apple lawsuit. They won on 8 and lost one 1. (The least important one) That's life in corporate America. I could probably go down the list of fortune 100 companies and find something, anything they did and received a judgement in a court of law. The punishment for Apple, they now have to allow verbiage saying where the service can be purchased elsewhere.
After all of these posts, no one will ever change anyones' mind. The anti-trust, pro-DMA group hails the legislation, while the folks who want less, not more, governmental regulations decry the regulation.
Just a typical day on the MR website.