How to rewrite a url without a query parameter doubling its value?

I need the following URL:

http://example.org/sessionstuff/kees/view.aspx?contentid=4&itemid=5

It needs to be rewritten so that it goes to:

http://example.org/sessionstuff/view.aspx?site=kees&contentid=4&itemid=5

Basically this will take a value keesand put it as a parameter site. I am using the IIS URL rewriter module that uses the rules in my web.config. I added the following code to my web.config:

<rule name="RedirectSite" stopProcessing="true">
    <match url="^(\D[^/]*)/(.*)$" />
    <action type="Rewrite" url="{R:2}?site={R:1}" />
</rule>

Everything works fine, but when I do the postback, the parameter sitedoubles. I checked this using the following code on my .aspx page:

<h3>Querystring</h3>
<ul>
    <% foreach (string key in Request.QueryString.Keys)
        Response.Write(String.Format("<li><label>{0}:</label>{1}</li>", key, Request.QueryString[key])); %>
</ul>
<asp:Button runat="server" Text="Postback" />

First time

Querystring
site: kees
contentid: 4
itemid: 5

Second time

Querystring
site: kees, kees <--- double
contentid: 4
itemid: 5

site ? .

_{: , appendQueryString="false" . >}