There is no duplication. It's exactly the same logic as Category:Film and Category:Films, with the top level category grouping much more than just films. Here, Category:Documentary film includes sub-categories for documentary filmmakers, documentary film awards, documentary film festivals, etc., along with documentary films. Shawn in Montreal (talk) 22:37, 16 April 2010 (UTC)[reply]
Thank you for clearing that up. DougHill (talk) 00:15, 17 April 2010 (UTC)[reply]
I had the same question myself at first. It's a little confusing when you first come across it: even more so for the usage of either Cinema or Film as a top-level category. Cinema of FOO is the only acceptable structure for geographical categories, while FOOian film is used for ethnic category branches. I learned that one the hard way. best, Shawn in Montreal (talk) 01:25, 17 April 2010 (UTC)[reply]