Duplicate content refers to blocks of text that are heavily repeated throughout multiple pages in a website. If you wish for your site to appear at the top of the rankings in the elite search engines, you should be aware that search engines will often filter out sites that have duplicate content. They due this in order to ensure that there is only one version of the content to be found in their index, so that they can be sure that a site is not trying to fool a search engine to receive a higher ranking. Creating duplicate content on your site is like telling a search engine that you want competition with yourself. A search engine will penalize you for this, thus diminishing your traffic and hurting your ranking.
One good way of avoiding this is minimizing boilerplate repetition. Instead of having an extensive block of text of numerous pages, you should create a short summary and a link on each page that will refer back to the block of text. Another good way of avoiding duplicate content is to manage your content system effectively by ensuring that copy is not duplicated in different areas of your site, including blogs, homepages and achieved pages.
Also, make sure to keep your internal linking system consistent. Keep your URL concise and if you do wish to retain duplicate content, ensure that your links within that content refer back to the same URL. If you wish to move your site to another location or have two domains with the same content, be sure to use a 301 redirect. This allows search engines and web browsers to re-index your site and theoretically retain your ranking.
An illustrated guide on understanding the process of how a search engine scans duplicate content can be found at SEOmoz.