title: magento $order->getAllItems() return twice the same item
body: <p>I'm writing an observer that check every item in a order, at some point I get the items</p>
  
  <pre><code>  foreach($order-&gt;getAllItems() as $item){
      //do something
      echo $item-&gt;getSku();
    }
  
    //output
    sku-first
    sku-first
    sku-second
    sku-second
  </code></pre>
  
  <p>but I get twice the same item with the same sku of course, where's the catch? maybe in some configuration file?</p>