In both Yahoo and Google, canonical URLs can sometimes become an issue. A canonical URL issue is one where the search engine has to figure out that several different URLs are actually the same page and which one of those URLs is the best one to index. Unfortunately, the search engines frequently are unable to do this correctly without some help from the site owner.
The best way to make sure that you don’t have a canonical issue is to fix canonical URLs before the search engines find them. The most common canonical URL potential problem is the www versus the non-www version of your website.
To a search engine,
http://www.example.com and
http://example.com are two different URLs with the exact same content. The search engines will suppress one version or even both versions because, as far as they are concerned, they are duplicate content. If search engines index all of the page on your site like this, your site could suffer serious rankings loss in the search results.If your website runs on a Unix server, you can fix canonical issues by redirecting with .htaccess files. You simply will be redirecting the non-preferred version of your URLs to the preferred version.