Duplicate content is one of the most discussed, blogged, and talked about SEO topics - well, after link building, of course. Based on Google's webmaster guidelines, "Duplicate content generally refers to substantive blocks of content within or across domains that either completely matches other content or is appreciably similar. Most of the time when we see this, it's unintentional, or at least not malicious in origin."
What exactly do "substantive" and "appreciably similar" mean? In my view, two pages can be termed "duplicate" if 30 percent or more of the page elements - title, URL, content - are similar to each other. For example, in the news/blog world, articles are often syndicated across numerous websites.
Two types of duplicate content exist: within a domain and cross-domain.
Within a Domain
This kind of duplicate content arises within the same site or domain. The most common example is a scenario like abc.com, where http://abc.com, http://www.abc.com/index.html, and http://www.abc.com all point to the same page. The solution here is simple: 301 redirect http://abc.com and http://www.abc.com/index.html to http://www.abc.com.
Duplicate content issues can also arise when the crawler can get to the same piece of content through two or more different paths. Example: shopzilla.com/digital-cameras/402/canon/259-43010/products and shopzilla.com/digital-cameras/canon+digital+cameras/402/products are very similar. In such cases, Google picks one and discards the other. If you have tons of these types of instances on your site, I suggest using a canonical tag - pick a URL that's already indexed by Google or is more relevant to users, and have the second URL point to the primary using the canonical tag. This will help in two ways:
Here are a few common scenarios where duplicate content arises between domains.
Content syndications: This usually happens when one domain syndicates its content to another domain. The example below shows CNET's content syndicated on nytimes.com. According to Google, "If you syndicate your content on other sites, Google will always show the version we think is most appropriate for users in each given search, which may or may not be the version you'd prefer."
In most instances, "most appropriate for users" corresponds to greater page authority. In this case, CNET shows up in the number two spot for the query "Canon PowerShot S90," while nytimes.com is on page three of the results.
Affiliates and co-brands: Both affiliate and co-brand deals result in duplicate content issues if not done correctly. Although co-brand deals are generally a thing of the past, they do still occur. From an SEO perspective, I would stay very far away from co-brand deals, because they inevitably result in one of the sites being completely removed from the search engine results pages (SERPs).
Here are some things to keep in mind when syndicating your content or starting an affiliate program.
Removing duplicate content from your own domain is easy, so complete that as soon as you can. Duplicate content across domains is a totally different beast, but if you play by the rules, in most cases, you should be able to tame this beast.
This column was originally published in SES Magazine in July 2010.
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Prashant Puri is head of global SEO for Shopping.com (an eBay Inc Company). He is responsible for SEO for sites that run across five countries. He has more than eight years of online marketing experience, including stints at Yahoo and AT&T. He's built numerous sites into multimillion unique visitor sites through a combination of SEO and SEM.