If your website have multiple URLs with same contents web spider do not crawl all URLs.
Let's see how web spider index page URL. If you have page with multiple url and with same contents.
Like, http://www.yoursite.com/page.aspx?id=90 or http://www.yoursite.com/page.aspx?id=91
Web search spider will index http://www.yoursite.com/page.aspx not id wise page.(Query String)
Now,All major web spider Google,Yahoo and Microsoft allows you to specify your preferred version of a URL.For that you have to add new canonical tag in page header section.
Like,<LINK href="http://www.yoursite.com/page.aspx?id=90" rel=canonical>
For Dynamically Adding this canonical tag in ASP.NET.
Dim cssLink As New HtmlLink()
cssLink.Href = http://www.yoursite.com/page.aspx?id=90"
cssLink.Attributes.Add("rel", "canonical")
Me.Header.Controls.Add(cssLink)
For more details Google,Yahoo and Microsoft
Canonical URL Tag - The Most Important Advancement in SEO Practices Since Sitemaps