title: Magento 2 add custom field to sales order export csv issue
body: <p>I have added a tracking number column in sales order grid. It is displaying fine in Sales order grid. But while I export the data by csv the following error has occurred.</p>
  
  <blockquote>
    <p><strong>You cannot define a correlation name 'sst' more than once</strong></p>
  </blockquote>
  
  <h2>The following steps I have implemented.</h2>
  
  <p>Added the custom column into sales_order_grid.xml</p>
  
  <pre><code>&lt;listing xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Ui:etc/ui_configuration.xsd"&gt;
      &lt;columns name="sales_order_columns"&gt;
          &lt;column name="track_number"&gt;
              &lt;argument name="data" xsi:type="array"&gt;
                  &lt;item name="config" xsi:type="array"&gt;                   
                      &lt;item name="filter" xsi:type="string"&gt;text&lt;/item&gt;                    
                      &lt;item name="label" xsi:type="string" translate="true"&gt;Tracking Number
                      &lt;/item&gt;
                  &lt;/item&gt;
              &lt;/argument&gt;
          &lt;/column&gt;
      &lt;/columns&gt;
  &lt;/listing&gt;
  </code></pre>
  
  <p>And override the <strong>sales_order_additional_columns</strong> function by plugin in di.xml file</p>
  
  <pre><code>&lt;config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd"&gt;   
      &lt;type name="Magento\Framework\View\Element\UiComponent\DataProvider\CollectionFactory"&gt;
          &lt;plugin name="sales_order_additional_columns" type="Vendor\ModuleName\Plugins\AddColumnsSalesOrderGridCollection" sortOrder="100" disabled="false" /&gt;
      &lt;/type&gt;
  &lt;/config&gt;
  </code></pre>
  
  <p>Overrite  the Collection of sales order grid.</p>
  
  <pre><code>&lt;?php 
  namespace Vendor\Modulename\Plugins;
  
  use Magento\Framework\Message\ManagerInterface as MessageManager;
  use Magento\Sales\Model\ResourceModel\Order\Grid\Collection as SalesOrderGridCollection;
  
  class AddColumnsSalesOrderGridCollection
  {
      private $messageManager;
      private $collection;
  
      public function __construct(MessageManager $messageManager,
          SalesOrderGridCollection $collection
      ) {
  
          $this-&gt;messageManager = $messageManager;
          $this-&gt;collection = $collection;
      }
  
      public function aroundGetReport(
          \Magento\Framework\View\Element\UiComponent\DataProvider\CollectionFactory $subject,
          \Closure $proceed,
          $requestName    
      ) {        
          $result = $proceed($requestName);
          if ($requestName == 'sales_order_grid_data_source') {
              if ($result instanceof $this-&gt;collection
              ) { 
                  $select = $this-&gt;collection-&gt;getSelect();
                  $select-&gt;join(
                      ["sst" =&gt; "sales_shipment_track"],
                      'main_table.entity_id = sst.entity_id',
                      'sst.track_number'
                  )
                      -&gt;distinct();                                   
              }
  
          }
          return $this-&gt;collection;
      }
  }
  </code></pre>