title: Avoid that script in MasterPage and in Iframe child page is executed twice
body: <p>I have developed a script with jQuery that everytime a <code>&lt;select&gt;</code> selection is changed then some logic is executed. </p>
  
  <p>Initially I added that script to the <code>MasterPage</code>, in order to be sure that every page would be considered - however, I have some external pages (<code>aspx</code>) that do not use the <code>MasterPage</code> and, as consequence, don't have the script. </p>
  
  <p>To solve this I added the same script to a custom control, that is rendered as a <code>&lt;select&gt;</code> and is also present in those external pages. Unfortunately in this way the script is executed twice, instead of just once.</p>
  
  <p>How can I change my code in order to ensure that the script would be executed in every page, but just once?</p>