Old Norse dueling, the Holmgang, was outlawed when griefers turned it into a cynical thing, challenging weaker neighbors over trivia so they could kill them and take their land, legally.
But then people realized you could have someone fight for you if they'd swear that your claim was true (IE, they'd be a witness). If you lost the duel, it would mean that you lied, making it perjury. So your equivalent of a lawyer would be some renowned fighter, undefeated in combat (since if you were defeated, you wouldn't be able to swear in as a witness).
It turned into situation where you'd use the person you hired as a negotiation tool (oh you hired "Bob the Barbarian" as your champion? Let's settle 60/40), so most challenges wouldn't even come down to a fight.
In the book, the legality of this form of dueling seems to be justified by an absolute need for strength, and the survival of (only) the fittest, in order to adapt to the hostile environment that they live in.
When physical superiority stops being a necessity for the survival of the community, this type of dueling law becomes a problem.
Anyway though wasn't a major theme of the back half of that book that this practice was always wasteful and unbeneficial. That there is no situation where a community becomes stronger by losing a cunning leader to merely a stronger fighter. That this was possibly one of the ways their culture had been manipulated to make them fragmented and susceptible to control.
Yes, towards the end of the book Paul convinces the tribe to let go of the old "way" so that they can be as strong as possible for the (imminent) battle. They allow him to become their leader without killing Stilgar, the previous leader.
However, the book makes it very clear that the totally ruthless environment, culture, and way of life on Arrakis (and on Salusa Secundus) is the cause of the Fremen (and Sardaukar) making such vastly superior soldiers.
But of course that only matters over long stretches of time. Putting your infighting ways on hold temporarily when you are gearing up for a massive offensive would always make sense. "Do you smash your knife before battle?" Paul asks in his speech. So I don't think the point is that it "was always wasteful and unbeneficial", but that even if the ruthless duels were good for the tribe's fitness in the long term, right then it was more important that in the short term they needed all the strength they could get.
Yes but it's fiction. That's possibly true within the fictional world of the book because the author decided it would be.
In actual history that view is disproved time and time again and historical consensus view is that societies that are more interpersonally brutal and competitive are not stronger for it. See HN-favorite historian bret devereaux's 40,000 word essay on exactly this subject, using exactly the fremen as the focal point.
Oh I don't dispute whatever it is the historians are saying at all. I was simply arguing about what the themes of the book were, since that was what you were also talking about in your previous comment. Whether the themes of the book are scientific or not I don't know much about at all.
So I guess what you are saying is that Frank Herbert might have a more or less inaccurate take on social/political/historical science, which would not surprise me.