While creating duplicate content intentionally as a means to bolster search engine rankings has long been punished by google, some people are being penalized for duplicate content would did not know they were commiting the offense. This happens when you use the same web content on multiple or sub-domains. If this doesn’t make sense, essentially, if you have www.website.com, www.subdomain.website.com, and subdomain.website.com, you could (not always) unknowingly be penalized in google, and other search engine, ranking.
Google has come up with a nifty way to avoid this. In their webmaster tools, you can submit a preferred URL by which your site will be indexed. This means that rather than going through all your (repeated) content, they will index your preferred version, which will keep search results consistent, and save you from getting penalized. Google spells this out and more details on their Webmaster Blog.
