WordPress has a built in feature that you really want to take advantage of - Optional Excerpts.

Why? By writing a unique sentence or two in this field you can minimize the duplicate content created by your category pages.

How? WordPress creates a category page by pulling the title and first few lines of each post in that category and displaying them all on one page. This is essentially cut and stitched duplicate content from elsewhere on your site.

Why Do We Care? If you’ve set them up right, your category pages are very well themed about one particular topic – blogging for example. They contain good keywords and link out using good keywords to other pages on the blog. All of these things can help with their potential for ranking and make them some of the most powerful pages on your blog. However, if they are flagged as duplicate content and filtered out of the SERPs, that potential is lost.

How Do Optional Excerpts Help? Very simply, they eliminate the duplication of content by displaying the alternate descriptions you write for each post instead of the first few lines of the posts.

Caveats? If you place any post in more than one category, then the same optional excerpt for it will be displayed on all of the category pages in which it is placed. This, again, creates duplicate content. Therefore, try to restrain from placing a post in more than one category on your blog. You can do this by either creating many highly-focused categories or a small number of categories that can be catch-alls for a variety of topics.

If you don’t write something in the Optional Excerpt field for a post, then WordPress defaults to pulling the content it displays from the beginning of the post, so a little laziness or forgetfulness can result in a lot of duplicate content on your blog.