After upgrading to Magento 1.9.3.10 I kept getting the following error when trying to go to the Customers screen. A similar error can also happen on other screens. It's when a Collection is being built, BUT there are duplicate entity_ids in the SQL results. To fix it, turn off the throw error.
Item (Mage_Customer_Model_Customer) with the same id "XXXX" already exist Edit lib/Varien/Data/Collection.php
Find the addItem function and comment out the "throw" line :
public function addItem(Varien_Object $item)
{
$itemId = $this->_getItemId($item);
if (!is_null($itemId)) {
if (isset($this->_items[$itemId])) {
// throw new Exception('Item ('.get_class($item).') with the same id "'.$item->getId().'" already exist');
} else {
$this->_items[$itemId] = $item;
}
}
else {
$this->_items[] = $item;
}
return $this;
}
DONE! Got this from https://support.webcto.eu/index.php/Knowledgebase/Article/View/50/8/item-mage_catalog_model_product-with-the-same-id-already-exist