Canonicalization and Consistency – Uniformity in URLs – SEO Best Practice Snippets
Canonicalization issues are those relating to selecting a uniform, primary version of URL to use when more than one URL can reach the same location or document.
It is important to decide on one single version that will be used uniformly throughout all your endeavours. Then all alternate versions should be set to have a server-side permanent redirect, (a 301 redirect, see later section), in place, so that if any inadvertent links to another version are out there, the browser and/or spider will be moved to the correct version. This helps to ensure that search engines will index only the primary, counting all inbound links to other versions towards the primary URL more correctly.