The preferred URL for indexing in cases of duplicate (or highly similar) content or where more than one URL can be used to access the same content (for instance www.mydomain.com and mydomain.com).
Since duplicate content or the appearance of duplicate content can negatively affect search engine rankings or lead to the “wrong” page being indexed, specifying a canonical URL is a good practice.
The canonical URL for a page can be specified by including a <link> element in the <head> of the page as follows:
<link rel="canonical" href=http://www.mydomain.com/canonical.html />