SEO Factors: On the Uniqueness of Document Text
When it comes to optimizing your site (and getting a higher rank on search result pages), nothing, and we mean nothing, is more important than the content that you make available on your domain. More importantly though, the uniqueness of that content is increasingly playing a significant role in determining rankings by search engines. Featuring content that is duplicated elsewhere on your site (or on another website) could cause that page's value to be decreased, or even worse, removed entirely.
What is Duplicate Content?
Websites with identical pages, sites which "scrape" content, featuring similar product descriptions from merchants (most often used by affiliates, distribution of articles in article directories- this is what is meant by duplicate content and are really the most egregious violations. Duplicate content filters are often modified by the search engines and can cause significant changes in rankings for those abusing content. If you've written the content yourself, haven't poached the content from elsewhere, and are featuring that content on only page of your site, then don't worry about duplicate content penalties.
Many professional SEO's agree that if you can get around the duplicate content filters the chances of being penalized in the long run are slight - this means that in the end you're probably going to be just fine if you modify or feature additional copy about the duplicate content you are posting. For example, say you find a really great article at Website Services you'd like to republish on your site - a common occurrence. If you write a few paragraphs about why it's an important article, or why you agree or disagree with the points made in the article, you'll be able to get around most content filters - i.e. noone gets penalized.
Other posts in the SEO Factors series:
On the Importance of Title Tags
On the Importance of Internal Linking