Earlier this month a new <link> tag parameter became available with the purpose of providing search engines with the definitive URL for pages. I have created a (hugely) basic plugin which will generate these tags for you, so long as your site’s URL structure is completely segment-based. 99.99% of EE sites will be.
To see why you should bother with this see Google’s documentation, Matt Cutts’ writeup and SEOmoz, which describes it as ‘The Most Important Advancement in SEO Practices Since Sitemaps’.
To implement the plugin just upload it and add {exp:canonical_tag} within your <head> tags.
