User blog comment:Portgas D.Eros/Zoro and Sanji/@comment-1595565-20101004202739/@comment-2031609-20101004205852

If memory serves Ivankov knew about it when Luffy used it during the Whitebeard War. He didn't use it nor said that he knew how to use it, but he at least knew what it was. But that wouldn't really matter since he may not have ever actually taught Sanji since he was just supposed to go around looking for the 99 newkamas. Mihawk probably knows about it, though he was never seen using it or talking about it, but probably didn't teach it to Zoro. But yeah, I do pretty much agree that it would be boring if they knew it right off the bat.