"Access-Control-Allow-Origin" Response Header send twice via IIS
<p>I am using the <code>Access-Control-Allow-Origin</code> for enabling CORS on my site and have the following part added to my site web.config:</p>

<pre><code>&lt;httpProtocol&gt;
  &lt;customHeaders&gt;
    &lt;add name="Access-Control-Allow-Origin" value="*" /&gt;
  &lt;/customHeaders&gt;
&lt;/httpProtocol&gt;
</code></pre>

<p><strong>I have used the IIS GUI to change this setting!</strong></p>

<p><strong>There are no double entries in the web.config!</strong></p>

<p>But as soon as I load the page I see following:</p>

<p><a href="https://i.stack.imgur.com/uFnHh.png" rel="nofollow"><img src="https://i.stack.imgur.com/uFnHh.png" alt="Image"></a></p>

<p>My site is in the only node in the <code>Sites</code> folder and my server itself has no custom headers attached to it.</p>

<p><strong>But why am I getting this doubled response header?</strong></p>