A featured WebmasterWorld thread goes over the technical issues specifically with WordPress blogs and how they are very susceptible to duplicate content issues. But it is not just WordPress, it is most blog formats, even this one.
This single article I am writing right now can be found in a number of different pages all on the same site. You will be able to find this article on the home page, on the search engine optimization category page, on the 10/20/2006 page, on the October 2006 page, on the individual entry page. Also, if I placed this article in multiple categories, you would find the same content on those pages as well.
Is this a problem? I don't see it to be a huge problem. I have always been an advocate of Google figuring out the problem and not having webmasters figure it out, because most outside of the SEO world have no idea that this is an issue. And for the most part, I think for the most part they do a good job with blog software used widely.
But if you are worried, the WebmasterWorld thread has tips and other discussion on what to do.
Forum discussion at WebmasterWorld.
