When optimising your web page for search, you may come across canonicalization issues which could adversely affect your search engine performance. For this reason, you should be aware of any canonicalization issues and move to address any problems to ensure that your Search Engine Optimisation work is not carried out in vain.
In web search and search engine optimisation, URL canonicalization refers to the process of dealing with web content which has more than one possible URL, such as a www domain and a non-www domain.
For whatever reason your homepage may be accessed by both http://www.domain.com and http://domain.com, a subtle difference but a crucial one in SEO terms. Having multiple domains for the same web content can cause problems for search engines, such as determining which domain should be shown in the search results listings.
One of the criteria by which web pages are assessed by search engines is popularity. The more hits your page receives, the more relevant and useful it will be perceived by the search engines and thus it will rank favourably. One of the problems that can arise from multiple domains is that while web users would use both domains to access the same information, their performance statistics will be collected separately, potentially halving the sites popularity and ranking.
Decide which domain you wish to use and 301 redirect the other to it. So if http://www.domain .com is the domain you wish to use, 301 redirect http://domain.com to it. This will permanently divert the latter’s traffic to the domain of your choice and means that this domain will appear in search listings and benefit from all of the popularity and performance.