It used to be thought that duplicate content, something that Joomla is often guilty at creating, would earn you a penalty slap from Google. Those in the SEO community would often keep raising the issue as a concern, and Joomla dev’s would maintain there was nothing to see, move along.
The reality is more subtle, there is no duplicate content penalty in Google, but duplicate content is an issue yet unresolved in Joomla.
The reason for a duplicate content "penalty" was because of scrape sites and general SEO naughtiness. Engaging in this practice would earn you a Google slap. Ouch!
Google then improved its algorithm and now claim no slapping is needed. Hurrah! They spiders are clever enough to figure out what is where.
"When we detect duplicate content, such as through variations caused by URL parameters, we group the duplicate URLs into one cluster."
But not so fast Mr Joomla.
Duplicate content in Joomla is caused by issues in the SEF routing (third party extensions can be just as guilty). I most often see it when page does not have a menu item associated with it, for example when the link comes from a category view and then also a module like Latest News. You end up with two links to the same page, for example:
Google does not seem to be able to spot these as duplicate content, and has treated them as separate page, each earning page rank. The problem is not just duplicate content, its pagerank dilution. With a single URL for that page, my PR 4 and 5 would be combined, hopefully with a slight bump.
To fix this we need a single URL for a page regardless of where it might exist. An *easy* fix would be to allow an option for the URL structure to be just:
This would have the added (huge) bonus of not making all your inbound links you have worked hard to get become redundant when you move an article around in your site!
Do you have any pages with more than one pagerank in your site?