One of the most common duplicate-content-related issues in Magento comes in the form of layered-navigation-enabled category pages. When we set our categories to be "anchored" (see Chapter 2, Product and Category Page Optimization), we allow them to be filtered by certain product attributes.

Magento uses parameters within the URL in order to filter the product collection. The drawback of this is that sometimes these parameters can be cached as separate pages—leading to thousands upon thousands of duplicated category pages.

In an ideal world, the canonical element would take care of this problem, but sometimes our duplicate pages may still be cached by search engines (especially if the website has been live prior to turning on the canonical elements).

One of the most effective solutions to this problem is to use a bespoke function within Magento called an Observer function that will perform the following tasks: