title: mage registry key "_singleton/" already exists
body: <p>I know there are lot of posts with this problem, but I guess each of them is with different roots of it (at least from what I checked - nothing helped me).</p>
  
  <p>I am trying to fire the event upon click on a button from the user, but I get the upper mentioned exception in a browser alert <code>Mage registry key "_singleton/" already exists</code>.</p>
  
  <p>The part of the config.xml:</p>
  
  <pre><code>.....
       &lt;models&gt;
          &lt;packagecustomernumber&gt;
              &lt;class&gt;Package_CustomerNumber_Model&lt;/class&gt;
          &lt;/packagecustomernumber&gt;
      &lt;/models&gt;
  
  &lt;/global&gt;
  &lt;frontend&gt;
          &lt;events&gt;
              &lt;checkout_type_onepage_save_order&gt;
                  &lt;observers&gt;
                      &lt;type&gt;singleton&lt;/type&gt;
                      &lt;class&gt;packageName/customernumber/observer&lt;/class&gt;
                      &lt;method&gt;setCustomerNumber&lt;/method&gt;
                  &lt;/observers&gt;
              &lt;/checkout_type_onepage_save_order&gt;
          &lt;/events&gt;
      &lt;/frontend&gt;
  </code></pre>
  
  <p>And the class itself:</p>
  
  <pre><code>class Package_CustomerNumber_Model_Observer
  {
      public function setCutomerNumber($observer)
      {
          die('setCutomerNumber');
      }
  }
  </code></pre>
  
  <p>The button which should fire the even it checking out/saving the order, so the event should be correct.</p>
  
  <p>Any suggestions ?</p>