Only add items to a dropdown list once
<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>
