In my two-year journey as a blogger, I have come across a number of articles on the issues related to Duplicate Content and how it effects the SEO. Some claim it to be a hoax, while others take it to be of much importance.
Google itself discusses the issue in detail at demystifying duplicate content penalty but still remains a little foggy on the measures that could be taken to prevent it.
However, from now onwards, the duplicate content issue can be dealt with full confidence by making use of Canonical link tags that allow you to publicly specify your preferred version of a URL. In this way, all the other URLs without Canonical link tags but having the same content are taken to be pointing towards the same preferred URL.
Lets take an example here. Imagine the preferred URL of a page on your website is something like;
and the same page can be accessed by entering the URL;
So now you can simply add the <link> tag to specify your preferred version;
<link rel="canonical" href="http://www.example.com/product.php?item=iphone" />
inside the <head> section of the duplicate content URLs and Google will understand that the duplicates all refer to the canonical URL.
It not only will work with Google, but with all major search engines such as MSN and Yahoo!. For WordPress users, a plugin for Canonical URLs is also available.
Make sure to read about this issue in more detail at head on to Google webmaster central blog.