Although it makes sense to enforce unique URLs for you web pages as Google imposes a penalty on suspected duplicate content, it’s not always possible to do so. Google has now introduced a way to specify the unique URL (canonical URL) of any web page by using a meta link tag.
Setting this tag is simple – you first decide which URL you wish to use, and then add this link tag to the <head> section of your HTML page.
<link rel="canonical" href="http://www.example.com/my-blog-post" />Google claims that this would not be treated as a directive, but a hint that would be honored strongly. Additional URL properties, like PageRank, would be transferred as well.