Duplicate content is one of those SEO issues that can be a real pain to deal with. While Google says it doesn't 'penalize' duplicate content unless the duplicate content is clearly the result of malicious behavior, that doesn't stop webmasters and SEOs from worrying about it.
While such worries may be overblown, there are legitimate reasons why duplicate content can become problematic.
Fortunately, Google, Yahoo and Microsoft have come together and agreed to respect a 'canonical tag' that enables webmasters to specify the canonical, or original, URL of the page.
The best part: implementing this tag is easy. By placing the following tag between the <head></head> tags, search engines will know what the canonical URL is:
<link rel="canonical" href="http://www.yourwebsite.com/some-page" />
So, for instance, if you have a page, http://www.yourwebsite.com/product-name/ whose content is also accessible through http://www.yourwebsite.com/new-products/product-name/ and http://www.yourwebsite.com/featured-products/product-name/, adding <link rel="canonical" href="http://www.yourwebsite.com/product-name" /> between the head tags on the pages that contain the duplicate content will ensure that the major search engines know which page is the original.
In the case of Google, PageRank and other juice-carrying characteristics are transferred to the page specified in the canonical URL.
While the use of canonical tags is not required and is not a directive, it's a hint that Google says it will "honor strongly".
Other Google-specific details:
- Both relative and absolute paths can be used.
- Content does not need to be 100% duplicate. Google understands that there may be slight differences between pages.
- Canonical tags can point to a URL that is a redirect. Google will try to follow the redirect as it normally would.
- Google is "lenient" when it comes to contradictory canonical tags but suggests that all tags be consistent for best results.
- Canonical tags can be used to point to URLs on a subdomain but Google will not respect canonical tags pointing to a different domain.
The fact that Google, Yahoo and Microsoft have all accepted the use of the canonical tag (a relatively rare occurrence), if your website has duplicate content issues, putting this to use looks to be worthwhile.