Google is trying to help webmasters with the duplicate content issue they have started a tag which can be placed in meta of your webpages head section which will tell the google bot that “Of all these pages with identical content, this page is the most useful. Please prioritize it in search results.”.We all do have many pages in our websites which are similiar to the other webpages well google bot knows that these pages have the same content, they may index only one version for search results. Thier algorithms select the page we think best answers the user’s query.

How to put canonical tag or how to specify the canonical page ?

To state a canonical link to the page

http://www.example.com/product.php?item=chirkut, create a
element as follows:

Copy this link into the section of all non-canonical versions of the page, such as http://www.example.com/product.php?item=chirkut&sort=price.

If you publish content on both http://www.example.com/product.php?item=chirkut and https://www.example.com/product.php?item=chirkut, you can specify the canonical version of the page. Create the
element:

Add this link to the section of https://www.example.comproduct.php?item=chirkut.

for more info read this http://google.com/support/webmasters/bin/answer.py?answer=139394