I know Duncan Idaho spent time with the Fremen but I can't find the name the Fremen called him by. I think it translated to He who fights with many blades.
I believe you're referring to the fanfic The Unfinished Universe by foxflowering (note, NSFW).
He's referred to in the story as;
"Keifusu al-Raimjal.” Paul glanced at Duncan, translating with a look of faint skepticism, “The man of many blades?”