"A canonical link element is an HTML element that helps webmasters prevent duplicate content issues by specifying the "canonical", or "preferred", version of a web page[1][2] as part of search engine optimization. It is described in RFC 6596, which went live in April 2012.
Duplicate content issues occur when the same content is accessible from multiple URLs.[3] For example, http://www.example.com/page.html would be considered by search engines to be an entirely different page to http://www.example.com/page.html?parameter=1, even though both URLs return the same content. Another example is essentially the same (tabular) content, but sorted differently."
Answer
This article has some useful information about canonical links.
Canonical link element
http://en.wikipedia.org/wiki/Canonical_link_element
"A canonical link element is an HTML element that helps webmasters prevent duplicate content issues by specifying the "canonical", or "preferred", version of a web page[1][2] as part of search engine optimization. It is described in RFC 6596, which went live in April 2012.
Duplicate content issues occur when the same content is accessible from multiple URLs.[3] For example, http://www.example.com/page.html would be considered by search engines to be an entirely different page to http://www.example.com/page.html?parameter=1, even though both URLs return the same content. Another example is essentially the same (tabular) content, but sorted differently."