How to use Canonical links with Joomla to avoid duplicate content

Many sites have a problem with duplicate entries in the Google index. This can happen when you use several links to the same page. For instance, a page might be referenced like this:

http://www.joomlablogger.net/about/

or like this:

http://www.joomlablogger.net/about/?source=google

The latter link is from Google Adwords, and contains a parameter and a value to tell the tracking software where the click came from (this is just a simplified example). This might seem unproblematic. However, since Google base their indexation upon following links from other sites, you might (and probably will) end up with two occurances of the same page in Google. Also, the fact that some people link to joomlablogger.net and others to www.joomlablogger.net, can lead to duplicate content in the search engines.

There are several ways to avoid duplicate cotent in the search engines. XML sitemaps, 301 redirects, linking consistently to www.yoursite.com or yoursite.com respectively, setting up Google Webmaster Tools correctly etc. However, sometimes you don't have full control over the server or other elements of the site.

Enter canonical links!

Canonical links (preffered links) are, as I mentioned, link tags that contain the preferred URL to the page. The link tag can look like this:

<link rel="canonical" href="http://www.joomlablogger.net/about/" />

Placed in the head area of your source code, the link tag tells Google what the correct URL to the page is. And voila: No more duplicate entries (it might take some time to see them disappear if you already have them, though). Google, Yahoo and Microsoft announced support for the canonical link element on February 12th 2009.
Matt Cutts from Google has written extensively on this subject, and here are some of his articles and videos describing it:

• Learn about the Canonical Link Element in 5 minutes
• Video: Canonical Link Element

So how to use Canonical Links with Joomla

The first thing is that you need to use the sh404SEF component for Search Engine Friendly (SEF) URLs.

When you have this in place, you need to install the Canonical Links plugin from Gnome on the Run. You can download it here. This creates the canonical link tag for your SEF URLs automatically.

It might also work with other SEF extensions, but I haven't tried that out.

Good luck on avoiding duplicate content!