title: Error : The entry 'asp:System.Web.UI:System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' has already been added
body: <p>We are working on a web application and the target deployment includes Windows 2003 64-bit. When we run the application on the 64-bit OS, it gives the following error:</p>
  
  <pre><code>The entry 'asp:System.Web.UI:System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' has already been added
  </code></pre>
  
  <p>The web.config file includes the following:</p>
  
  <pre><code>&lt;pages validateRequest="false" enableEventValidation="false" viewStateEncryptionMode="Never" theme="esl_theme"&gt;
      &lt;controls&gt;
          &lt;add tagPrefix="asp" namespace="System.Web.UI" assembly="System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/&gt;
      &lt;/controls&gt;
  &lt;/pages&gt;
  </code></pre>
  
  <p>And if we remove the line <code>&lt;add tagPrefix="asp" namespace="System.Web.UI" assembly="System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/&gt;</code>, it seems to work.</p>
  
  <p>Can somebody help to understand the problem and take corrective action? The application does not compile on 32-bit OS without this.</p>