Duplicate Content - Canonical URLs and Canonicalization - Search Engine Optimisation (SEO), SEO Australia
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Duplicate Content - Canonical URLs and Canonicalization

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It is, or should be, common knowledge that duplicate content does not do your website any favours, particularly where the major search engines such as Google, Yahoo and MSN (Live) are concerned.

What you may not know is that you can have one page, but multiple links going to that page which will also be doing your site "harm".

This situation may arise for example in a shopping cart, where one page may be linked to using two or more different URLs.

Canonical Perhaps you are selling a search engine optimisation book by the name "Mr Magoo's SEO How To Guide". The main URL to this page might be www.justweb.com.au/books/product.php?cat=seo&prod=seoguide, however, elsewhere in your site this page may be arrived at by the URLs www.justweb.com.au/books/product.php?prod=seoguide and www.justweb.com.au/books/product.php?prod=#123456 etc etc.

Essentially, this will be viewed as multiple pages with the same content. The actual link, or the one you want to be the "official" link, may not be the link that Google (or other search engines) decide to use. This process Google terms "canonicalization" - the act of picking the best (or most appropriate) link to display in it's index. Unfortunately, this may not be the link YOU want them to use.

Lets say different websites pick up different links to the same page. Google will give weight to the canonical link, and not the alternate links.

Up until now, this has been a problem, but easily remedied by using .htaccess rewrites, PHP rewrites, etc etc to attribute a 301 redirect.

Now, in what must be a first in cooperation, early in 2009 Google, Yahoo and MSN have agreed to a new standard. By simply adding a "link rel" tag to the header of your page, you can "suggest" the canonical link to the search engines. The tag to fix the example link above would look like:

<link rel="canonical" href="www.justweb.com.au/books/product.php?cat=seo&prod=seoguide" />

Google say that this is not actually a "directive", but instead a hint - one which Joachim Kupke, an engineer from Google's Indexing Team says they will honour strongly. This way, you can choose what you think is the most representative link for your page. EG, the link in the "link rel" example uses the keywords "seo", "books", and "seo guide" - all strong keywords.

Another example where the new canonical "link rel" tag may be very useful is in a situation where a business directory may link back to your pages using their own links, rather than yours. One directory that uses such referrer links is Clickfind™ - if you look at the inbound links to justweb, you will see they add a referrer link to their outbound URLs, however when you follow that link, notice the referrer has been removed by a redirect.

In my opinion, a 301 permanent redirect (which IS a directive) is the best, but the "link rel" may be easier to implement in some cases.

The difference between (eg) http://justweb.com.au and www.justweb.com.au is yet another example of a canonical link. You will notice with some websites if you use the http without the www, you end up going to the website at http://something, or even worse, nothing at all.

An example of this is the VoIP provider SIPME - go to http://sipme.com.au, and you are met with an "address not found" page, whereas www.sipme.com.au, or http://www.sipme.com.au works fine.

However, when you go to http://justweb.com.au and www.justweb.com.au, look at the address and you will see you are instantly at http://www.justweb.com.au. The same applies to deep links - go to http://www.justweb.com.au/searchengineoptimisation/duplicate-content.html (with www), and http://justweb.com.au/searchengineoptimisation/duplicate-content.html (no www) and you will see the URL is the same.

In summary, ensure your website URLs are always one or the other and do not have multiple variations, or in other words, ensure you have a procedure in place to force (or suggest) the canonical link.

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Rob - JustWeb

15.03.2009
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