If your Magento site has a lot of product attributes used for filtering products, also known as layered navigation, you might be concerned about duplicate content.
You may have noticed that when visitors click on the category filter in left hand navigation area, the url changes to include a category id, ie: ?cat=79.
This can cause quite a bit of confusion to the search engines, as there are at least two urls with the same content. Plus if you also select a color filter, the url will also add price filter, like so:cat=79&price=-400. There is a way around this dilema. It’s called a canonical tag, and of course Magento makes it pretty easy to implement.
After enabling the canonical tags, you will find that Magento has inserted a canonical tag in your page header. Here is an example tag:
This tag tells the search engines that http://mysite/red-widgets.html?cat=79 is a duplicate of the http://mysite/red-widgets.html page, and don’t include it in the index.
Like all things Magento, pretty easy if you know where to look.