Basically, Ive noticed this with a couple of my sites now. What I like to do is limit posts to one category each, but spread the tags out so that a couple undear each tag page. Well, I use long tail keywords in the title so I manage to come out somewhere in the first 5 pages as soon as google indexes each post, but when google starts to index the tag pages eg (tag/this/ and tag/that/) posts that appear in both tags will be exact copies of each other and google WILL punish you for duplicate content with the same content. You need to download teh All in One SEO pack for wordpress and choose "noindex tags". I like to keep my categories and archives indexed. I think this should work better. Let me know your results, it would be interesting to know how big teh effect is for removing multiple content.