There's that famous ad campaign from Wrigley's Gum with the tagline "double your pleasure". While doubling is often a great thing (ice cream scoops, salary, and fresh breath come to mind), it's not so great when it comes to doubling (or more) your site content, commonly known as "Duplicate Content".
It may seem logical to assume that the more places your content is listed, the better and more relevant your site may be. However, when there is more than one piece of identical content on the Internet, it is difficult for search engines to decide which version is more relevant to a given search query. To provide the best search experience, search engines will rarely show multiple, duplicate pieces of content and thus, are forced to choose which version is most likely to be the original (or best).
To determine if your content is duplicated, simply copy one or two sentences from the content in question and paste it into a search engine. If the exact same sentence(s) appear in the search results, then you have duplicate content.
Now what do you do? It depends on whether the duplicate content is on urls you own/manage or not. For instance: Let's say you own both www.phatblingbling.com and www.fatblingbling.com and you are trying to determine if you have duplicate content for www.phatblingbling.com. You notice that www.fatblingbling.com is one the search results, this is likely due to improper or missing 301 redirects. Simply implementing 301 redirects from www.fatblingbling.com to www.phatblingbling.com should correct this issue.
Now let's say you do not own the urls that appear in the search results, what do you do? Well, to begin with you will need to contact the webmaster and ask them to remove your content. 9 out of 10 times, the webmaster will take down the content.
And what if the search results return urls from your same domain? Well, there are multiple reasons this could occur, but the most common reasons for this have to do with Canonical (pronounced "cuh-nah-ni-kle") links, redirects, multilingual settings, url parameter configuration and bad content; which if that is the case, you have a bigger problem than duplicate content.
So, rather than doubling the amount of money you pay to clean up duplicate content, try these simple, cost-effective ways to eliminate most forms of duplicate content yourself. Who knows, maybe you could double the results, and "double the fun"!