Duplication in Content( Important ) In order to avoid duplicate content issues, we need to add the link tag inside the head section of the duplicate content URLs. It will be like: <link rel="canonical" href="http://www.xyz.com/product.php?item=red-widget" />
By duplicate content what exactly do you mean? Are you publishing the same page content on different websites? All your webpages should have unique meta tags on a specific site...
what he means is a url like: http://www.xyz.com/product.php?item=widget&color=red would be exactly the same page as: http://www.xyz.com/product.php?color=red&item=widget This may seem the same to the human visitor but the search engines will view this as 2 seperate pages and can cause duplicate content issues. This is particularly problematic within ecommerce websites.