One of the main problems with the Internet today is the amount of duplicate content. This poses problems for searchers who only want to see one version displayed in the search results. It causes problems for the search engines who do their best to index the original version. And finally it causes problems for Search Engine Optimisation (SEO) as site owners are concerned that if the search engines find multiple versions of a page their link credit will be worth less and as a result they will slide down the Search Engine rankings.
The top three Search Engines; Google, Yahoo, and Microsoft, have united in offering a way to avoid duplicate content clogging up the Search Engines and in doing so hope to make things easier for everyone. The solution lies in the “canonical tag”.