title: Only add items to a dropdown list once
body: <p>I have a dropdown list that is populated when the page loads, like this:</p>
  
  <pre><code>protected void Page_Load(object sender, EventArgs e)
  {
          ddlCars.Items.Add("Ford");
          ddlCars.Items.Add("Chevy");
          ddlCars.Items.Add("BMW");
          ddlCars.Items.Add("Jeep");
          ddlCars.Items.Add("Nissan");
  
  }
  </code></pre>
  
  <p>It works fine, but when a user changes the selection of the dropdown list, the list is repopulated and I see the same items twice, because the page is reloading and these values are being re-added.</p>
  
  <p>What would be the best way to stop this from happening? It is important that AutoPostBack stays enabled, so that I change information based on user selection.</p>
  
  <p>Thanks</p>