A canonical URL is the official or preferred version of a URL. If you have several pages on your website that contain nearly or completely identical content, those pages could be considered duplicate content and that duplicate content can become a serious problem that holds your website back in the eyes of the search engines.
Product Canonical URL Extension optimize precisely how your pages are referenced and indexed with fine-grained control over your canonical URLs, and avoid penalties for duplicated content.
By using Product Canonical URL Extension you get an option in Product editing to add Canonical URL field to add Canonical URL.
Currently its only for Product Pages, but will be upgraded with more Options of Category & other CMS pages.
Instructions to Configure
1. Login to your Magento Connect and upload Product_Canonical_Url-184.108.40.206.tgz file.
2. After successful installation you will get a Top Navigation Vivacity Extension–>Product Canonical , by clicking on it you will get configuration page, where you can enable this module.
3. Edit any of your Product and in Meta Information Tab you will get a Product Canonical Input field to put any URL that you want to be canonical of current Product URL.
Note: If after installation you will get 404 error, please logout and login again, this problem will be resolved. This happens due to caching of Magento.