Most of the time webmasters and blogger are worried about the duplicate contents of their websites for search engines as with dynamic URLs it is possible to have more than one URL for 100% same content.

http://www.example.com/item.php?item=nokia-5800
http://www.example.com/nokia-5800
http://www.example.com/item.php?item=5800

I have already written some tips on this for wordpress

Thanks to Goolge as they have introduced the rel=”canonical” and by using this approach we can define the main or base URL for our content. Now it does not matter how search engine reach this page as it will always look at the rel=”canonical” tags and will recognize which is the actual parent URL and will index and  give all credit to that particular parent or main URL . This link-tag is  supported by Google.com Ask.com, Microsoft Live Search and Yahoo!.

Here is a sample tag for the above example

<link rel=”canonical” href=”http://www.example.com/nokia-5800” />

Now it does not matter how search engine have reached your web page it will always treat http://www.example.com/nokia-5800 as your main parent url. It’s a hint that search engine honor strongly.

Here is a google official video on this topic