We’ve talked quite a bit about duplicate content already, but what happens if you have an entirely duplicate site and don’t even know it? This actually happens to most people, and they have no idea. What happens is you have one version of your website, and then you have another, but Google can’t tell the difference because, hey, it’s still a robot. It’s import to let them know which one is the real version of your site.
Defining the “real” version of a website is known as canonicalization. In other words, your site can only live in one place. If your site can be found with the “www” at www.website.com, as well as without the “www” at website.com, your site has a canonical error. This means that there are duplicate sites, which search engines view as duplicate content.
In other words, tell Google which one is the real one!