Kahanamoku is one of those signatures that is out there, but many times terribly overpriced. I have owned three and currently have one signed at the 1932 Olympics. My favorite one of his was one that was completely missed by the seller. It was a steamship menu from 1932. Many of the old menus would have a page to get autographs, presumably of your table mates. I saw one on ebay and happened to look at the listing. Duke's signature was on that page. It was the ship he took to Los Angeles where he was an alternate on the U.S. water polo team which won a bronze medal.
From the steamship program:
Kahanamoku, Duke (1932) - 1.jpg
From a 1932 Olympic scrapbook that had over 200 signatures in it:
Kahanamoku,-Duke-(1932)---2.jpg
I still have the pages with 62 U.S athletes including Babe Didrikson and some medalists from the 1912 and 1924 games.