Say I have a category structure that looks like this:
a<br />
--a1<br />
--a2<br />
b<br />
etc
(a is the parent of a1 and a2)
Unless I have a post that is actually in category a (e.g., it's not enough to be in either a>a1, a>a2 or both) then the categories widget will not show category a or any of its subcategories.
So to make those categories show up I have to have at least one post that is in category a proper. Say that this post is categorized as being in a and a>a1. Then if I go to
subdomain.wp.com/tag/a
my post will show up twice. Once for being in a and once for being in a>a1.
It seems to me that these behaviors are contradictory. Either the categories should display on the sidebar regardless of whether there are any posts in a proper (so WP recognizes a>a1 as being part of a), or the post should only appear once when i go to /tag/a (so WP considers a and a>a1 to be exclusive).
This makes me think that the posts are retrieved by looping through the category and then all the subcategories. If that's the case, can it be changed so that if a given post in an appropriate subcategory is also posted to the category proper then it will be skipped the 2nd, 3rd, etc. times through?