When your website has been up for a couple of years like ours has, somethimes the natural evolution of the site can create more than one version of the same page, what does that mean? Well for example depending on the structure of your site the URL itself maye be a string of characters, have code like icat in it or may have been altered in some way by the use of a mod rewrite, (where URL’s are changed in to Google friendly versions of the same page). Take a page such as :
You may have a modrewrite in place that changes this to:
As far as Google is concerned both these URL’s are duplicates of the same page content and the ranking in Google’s organic search will be negatively impacted.
By using a ‘Canonical tag’ (check out the page source of this page to see it) you are telling Google which version of the page to index. All of the ranking weight of the 1st URL will be combined with the second URL – ultimately the 1st URL will drop from Google and the weight will all be on the 2nd. The canonical code is put before the /head section in the code of both pages – (all duplicates). For large php sites this code can be dynmically written so that it appears on all duplicate pages specfying the page that Google must index.
You can read more about Canonical Tags here: http://googlewebmastercentral.blogspot.com/2009/02/specify-your-canonical.html
Thats my tip for the day – have a great weekend!