Fix multiple category dupe bugs.

- When a parent has multiple categories, it will render its child
  multiple times. If its child has the same categories (which it will if
  there are *no* child categories), then it will render again for each
  category that the parent contains. (IE, if a parent has the categories A,B, when A is
  rendered, the child will render A,B and when B is rendered, the child
  will render A,B, thus duplicating the child in both A and B.)
- The fix here is to never permit the same doc_block to render content
  more than once in any given output file. N.B. This doesn't prevent
  end-users from duplicating doc blocks manually in the source, but this
  behavior has always been the case.
- Fix an instance where parent categories weren't correctly propagated
  to child categories, which might've hidden this bug in many
  cases.

Signed-off-by: Rajan Agaskar <rajan@pivotallabs.com>