Duplicate key value pair querystring

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ASP.NET

Hi,
I am using url re-write in my asp.net application web.config file.But when i submit a form the query string value has to be duplicated.how can i avoid this
Url Rewrite Code:

<rewrite>
     <rules>
        <rule name="Rewrite to Activate.aspx">
         <match url="^activate/([0-9.a-z]+)" ignoreCase="true"  />
         <action type="Rewrite" url="/activate.aspx?id={R:1}" />
       </rule>
     </rules>
</rewrite>

When i submit the activate button i got the error
http://www.sample.com/activate.aspx?id=100&id=100. Error Message:Input string was not in a correct format.
Id got duplicated...
Please help me to solve this.