Duplicate content (and its associated penalties/issues) may be a thing of the past now that the three major search engines in Yahoo!, Google, and Microsoft now support a new HTML tag, the tag, which reduces URL duplicates by documenting the preferred URL form to access each page no matter how it is retrieved. In layman's terms, you can now tell the search engines which URL it should have for the current page. If you're concerned that search engines aren't indexing all of your content, using the tag may benefit your SEO efforts greatly.
As it stands, search engines do a fine job of determining the correct URL, but usually only when the page redirects correctly. The redirects can be messy for dynamic sites that pass multiple parameters, for example, session IDs and other forms of variance. When you use the tag, you can indicate the canonical URL form for crawlers to use for each page of content, no matter how it was retrieved. This puts the preferred URL form with the content so that it is always available to the crawler.
Say for example that you notice the following URLs have been indexed:
https://www.example.com/products?trackingid=feed
https://www.example.com/products?sessionid=hgjkeor2
https://www.example.com/products?printable=yes&trackingid=footer
Specifying a link tag in the section of your page content (as seen below) will indicate to the crawler that the URL is present and that it should be represented as the preferred canonical URL designated:
<link rel="canonical" href="https://www.example.com/products">
The consortium has provided a few technical details which are important to note:
- URL paths can be absolute or relative (though absolute paths are recommended).
- A tag can only point to a canonical URL form within the same domain and not across domains.
- The tag will be treated similarly to a 301 redirect, meaning that link references will be transferred.
- Canonical links will be considered erroneous and deferred if tags are not used as intended.
It's anyone's guess whether SEOs and Web professionals will latch onto the canonical link tag. Search engines will most likely adhere to it a majority of the time, but they do reserve the right to do what they want. Joost de Valk has written a post on canonical URL links and provided some plugins for Wordpress, Magento and Drupal.