Doubt they’ll be shirtless for the fighting scene anyway. And why would we expect them to?
”Jamis stepped into the ring, slipped out of his robe and tossed it to someone in the crowd. He stood there in a cloudy gray slickness of stillsuit that was patched and marked by tucks and gathers. For a moment, he bent with his mouth to his shoulder, drinking from a catchpocket tube. Presently he straightened, peeled off and detached the suit, handed it carefully into the crowd. He stood waiting, clad in loincloth and some tight fabric over his feet, a crysknife in his right hand."
Dune, page 240, Chilton first edition 1965
Paul was also in a loincloth during the fight. They did it in the SciFi adaptation: