lib_Varien_Data_Collection.php

# line 367, magento-1.5.0.1
   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');
            }
            $this->_items[$itemId] = $item;
        } else {
            $this->_items[] = $item;
        }
        return $this;
    }