title: Ionic.Zip ArgumentException (An item with the same key has already been added)
body: <p>I'm using Ionic Zip to zip all folders in a specific folder excluding the backup folder (which created backups goes).</p>
  
  <p>This is my code:</p>
  
  <pre><code>ZipFile zip = new ZipFile();
  string mainpath = HttpContext.Current.Server.MapPath("~/");
  Directory.GetDirectories(mainpath).Where(d=&gt; !d.ToLower().EndsWith("backup")).ToList()
  .ForEach(d=&gt; zip.AddDirectory(d));
  </code></pre>
  
  <p>but after adding some directories, I'm getting the following error:</p>
  
  <blockquote>
    <p>An item with the same key has already been added.</p>
  </blockquote>
  
  <p>How is it possible? How can possibly be duplicate names in the folder list in the same parent folder?</p>