Google, Microsoft and Yahoo announced support for a new "canonical" link element in the header of a page that will help clean-up duplicate url's that have been indexed from your site.
The canonical link for this page, for example, would be:
<link rel="canonical" href="http://guyellisrocks.com/coding/adding-a-canonical-link-element-in-asp-net/">
To add this link tag to your header tag programatically using C# add the following to your Page_Load() function:
protected void Page_Load(object sender, EventArgs e)
{
HtmlLink link = new HtmlLink();
link.Attributes.Add("rel", "canonical");
link.Attributes.Add("href", "http://guyellisrocks.com/coding/adding-a-canonical-link-element-in-asp-net/");
this.Header.Controls.Add(link);
}