Using WordPress, I like to tag my posts in multiple categories to help with SEO exposure. However, after upgrading to 2.1 I found that my posts were showing up multiple times on the index, once for each category they were in.
Turns out the reason for this was that I was using Category Visibility-RH. This plugin alters the visibility settings for different categories. I have my delicious bookmarks posted to my blog and have these in a invisible category so they don’t junk up the primary index page. Turns out this plug-in wasn’t 2.1 ready. Fortunately Rich Hamilton, the author released a beta patch that is working perfectly for me. If you’re having this same problem definitely try out the beta patch. If you’re not into betas then you’ll want to disable this plug-in until it’s released RTM.