I think about this a lot and we've hacked a lot of blog scripts to avoid just such a problem. It really depends on a few things. 1. Do you have lots of other posts on those category pages? 2. Hopefully, the actual post itself is only accessible via ONE url. 3. Do you preview a paragraph or two, or the whole blog post on the category page? 4. Do you also get a lot of comments on your blog posts? If the answer is yest to 1, 2, and 4 you're probably fine. Also, if you're only showing a preview paragraph or two then you should also be fine. If the answer to any of those is no, IMO you can run into some problems but nothing that's gonna cause a penalty---possibly just lower rankings than you would otherwise have.