Duplicate Content has long been an issue with webmasters and is cause for concern because duplicating content dilutes your ranking in search results. Perhaps the most common cause of having duplicate content is in allowing both www. and non www versions of your site to exist. However, another place where I see duplicate content occur is when a customer has multiple domain names pointing to the same content. When you use multiple domain names, and there are good reasons to do this, you should pick a preferred URL and ensure that any secondary domains are 301 redirected to that URL.
I have previously blogged about how 301 redirects can prevent common problems from diluting the search engine ranking for your site. However, there are however a number of situations that create duplicate content concerns that are more difficult address. Webmasters now have additional tools to deal with duplicate content through Google’s Webmaster Tools panel. Through this addition to webmaster tools, Google Lets You Tell Them Which URL Parameters To Ignore. This will a great help, especially for many large dynamic sites.