title: "An entry with the same key already exists" appears when compiled under .NET 4
body: <p>I am trying to upgrade an ASP.NET application to .NET 4, but one page on my site contains an exception:</p>
  
  <blockquote>
    <p>Argument Exception: an entry with the same key already exists".</p>
  </blockquote>
  
  <p>What is different about ASP.NET 4 that might cause this problem?</p>
  
  <p><img src="https://i.stack.imgur.com/P5Fvk.png" alt="enter image description here"></p>
  
  <p><strong>One Solution</strong></p>
  
  <p>Not sure why but setting <code>clientIDMode="Predictable"</code> rather than <code>Static</code> seems to have avoided this exception message.</p>