Duplicate content is one of the most vexing and troublesome problems any website can face. Over the past few years, search engines have cracked down on pages with thin or duplicate content by assigning them lower rankings.
Canonicalization happens when two or more duplicate versions of a webpage appear on different URLs. This is very common with modern Content Management Systems. For example, you might offer a regular version of a page and a print-optimized version. Duplicate content can even appear on multiple websites. For search engines, this presents a big problem: which version of this content should they show to searchers? In SEO circles, this issue is often referred to as duplicate content, described in greater detail here.
The engines are picky about duplicate versions of a single piece of material. To provide the best searcher experience, they will rarely show multiple, duplicate pieces of content, and instead choose which version is most likely to be the original. The end result is all of your duplicate content could rank lower than it should.
Canonicalization is the practice of organizing your content in such a way that every unique piece has one, and only one, URL. If you leave multiple versions of content on a website (or websites), you might end up with a scenario like the one on the right: which diamond is the right one?
Instead, if the site owner took those three pages and 301-redirected them, the search engines would have only onestrong page to show in the listings from that site.
When multiple pages with the potential to rank well are combined into a single page, they not only stop competing with each other, but also create a stronger relevancy and popularity signal overall. This will positively impact your ability to rank well in the search engines.
Canonical Tag to the rescue!
A different option from the search engines, called the Canonical URL Tag, is another way to reduce instances of duplicate content on a single site and canonicalize to an individual URL. This can also be used across different websites, from one URL on one domain to a different URL on a different domain.
Use the canonical tag within the page that contains duplicate content. The target of the canonical tag points to the master URL that you want to rank for.
<link rel="canonical" href="https://moz.com/blog"/>This tells search engines that the page in question should be treated as though it were a copy of the URL https://moz.com/blog and that all of the link and content metrics the engines apply should flow back to that URL.
From an SEO perspective, the Canonical URL tag attribute is similar to a 301 redirect. In essence, you’re telling the engines that multiple pages should be considered as one (which a 301 does), but without actually redirecting visitors to the new URL. This has the added bonus of saving your development staff considerable heartache.
For more about different types of duplicate content, this post by Dr. Pete deserves special mention.