One of few legacy leftovers that was never dropped over the years is the common use of www domain prefix. It is not a problem per se for us humans, but SE bots have mind of their own. When traversing a non-www and a www version of a web site, apparently, google and other bots are treating them as two separate web entities, even if content is identical.
This causes confusion in SE databases and in a worst case a drop from the index.
To avoid this confusion, web site operators should redirect their non-www traffic to www version of the website, or redirect the www version to the non-www version of the site.
This should be done using 301 redirect, i.e. when request for http://example.com is made, the web server would return
HTTP/1.1 301 Moved Permanently
Location: http://www.example.com/
response with the location of the new site.
So how can I do this in IIS 7? Well, you have to edit web.config file, modify or add
the following section to its rewrite section.
<rewrite>
<rules>
<rule name="non www to www" enabled="true">
<match url="(.*)" />
<conditions>
<add input="{HTTP_HOST}" negate="true" pattern="^www\.([.a-zA-Z0-9]+)$" />
</conditions>
<action type="Redirect" url="<a href="http://www.{HTTP_HOST}/{R:0"
>http://www.{HTTP_HOST}/{R:0</a>}" appendQueryString="true" redirectType="Permanent" />
</rule>
</rules>
</rewrite>
This should be your first rule.
The rewite section should be placed in system.webServer section.
Then just refresh your site and voila you have all trafic to http://example.com/* redirected to http://www.example.com/*
I wish SE bot developers would build this logic into their robots, but instead millions of webmaster around the world have to make the above configuration manually 
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