In Switzerland we have a duopoly since forever, Coop vs Migros. Aldi and Lidl are also players but it is still mainly a duopoly. And that worked quite well, at least for the ~25 years I lived there.
Coop and Migros have the biggest margins in Europe. Hardly the picture of competition.
Seems more like tacit collusion.
Note that Switzerland is really a weird country, it’s difficult to compare it with other places.
And I always wondered if the goal is to provide service to customers/"owners" why are they expanding outside their geographical areas like to other countries.
Which is to say, a duopoly is never good.