Magento is a powerful, scalable eCommerce solution that allows site managers to control multiple store views from one administration panel. The ability to control multiple store views is definitely underestimated and undersold from a search engine optimization (SEO) perspective as store views can offer personalization and localization. Additionally, having control over a second store view can create opportunities for A/B testing and even allow a SEO to test URLs for categories and product pages.
Adding a new store view http://www.magentocommerce.com/knowledge-base/entry/creating-a-store-view in Magento can be done in four steps but before creating a new store view it is important to note that by creating multiple store views using the same catalog may introduce a new parameter to your URLs.
Normally, a new parameter only means the page with the parameter is a variation of the page without the parameter the Webmaster can control of the crawling of new parameters within Google Webmaster Tools (GWT). However, when a new store view is implemented there is a sneaky “___store” parameter that may be added to every URL on your website. The new parameter has just created a duplicate page for every page on the website and creates a very high risk of being penalized for duplicate content. Even if the site is not penalized for duplicate content the page with the new parameter will become indexed, cached, and shared with the incorrect or un-optimized URL. It is also worth to note all the new URLs will have a page authority of 1 and the duplication of your site content will wreck havoc on your search engine optimization efforts.
When a store view is initially installed the configuration settings are assigned to “default” and if a website only has a single store view the default value should never appear in the URLs of the site. Upon implementing a new store view the “___store” parameter with the value of default may be added to every URL within the original store view. This can be prevented if the “___store” parameter has not been commented out in the Magento Catalog widget.
If you are currently running multiple store views and are uncertain if this parameter exists on your domain using the Google operator site: and “___store” you can find out if the new store view is causing duplicate content. Here’s how to do it:
site:domain.com “___store” (that’s three underlines)
If you find that your pages have been indexed and duplicated with the “___store” parameter it may be recommended to use GWT to decide how Google should index your content. Caution: Use this method only if you’re sure how parameters work. Incorrectly excluding parameters could result in many pages disappearing from organic search results.