title: Magento Grid add new column
body: <pre><code>protected function _prepareCollection()
  {
      $collection = Mage::getResourceModel($this-&gt;_getCollectionClass());
      $collection-&gt;getSelect()-&gt;join('sales_flat_order_address', 'main_table.entity_id = sales_flat_order_address.parent_id',array('company'));
      $this-&gt;setCollection($collection);
  }
  </code></pre>
  
  <p>I use the above code to add a company field on order listing grid.
  but it shows "Item (Mage_Sales_Model_Order) with the same id "1038" already exist"</p>