Jodorowsky's Dune Uncovered
Scene 121-123 : Feyd Vs Farok
A title card tells us that it is 4 years later, and we are on Giedi Prime.
The Baron, and his guests of Count and Lady Fenring are watching Feyd fight the captured Fremen Farok.
Farok, driven by a madness from drinking elacca, is getting the better of Feyd, but as the blade approaches his neck Feyd shouts "Scum!" and Farok has been conditioned to freeze for a few seconds on this word, allowing Feyd to nick him with his poisoned dagger.
Farok is now almost unable to fight and Feyd turns his back allowing Farok one last attack, but Feyd easily avoids it and kills him with a flourish. The crowd roar "Hail Rautha!"
Later in the Harkonnen castle a meal is being served to the Baron, Feyd, the Count and Lady Fenring.
Fenring has realised that the "victory" was the result of conditioning and a poison dagger, setting Feyd up as a hero to replace the oppressive Rabban on Dune.
The Baron admits this is the plan, but Fenring warns that if the Baron attempts to enlist the 10 million Fremen as an army against the Emperor then Giedi Prime will be eliminated. To prove his innocence the Baron must kill all the Fremen!
That evening Lady Fenring seduces Feyd, conceiving a son that she and her husband hope will be the Kwisatz Haderach, allowing the Bene Guesserit to rule the galaxy, and the Fenrings to rule them!
From the Moebius storyboards we see that Feyd cheats in the same manner as in the script in his duel with Farok.
Also, from the presskit, is is clear the that the Fenring's plot to produce their own Kwisatz Haderach was part of the original script.
"Count Fenring, the Emperor's ambassador is plotting genetically : dominating his jealousy, he offers his wife to Feyd Rautha to produce the awaited prophet."