How do I change the default value of the “position” category attribute? in magento

You can change directly in this method Mage_Catalog_Model_Resource_Product::_saveCategories
Somewhere in there this is this:


$data[] = array(
    'category_id' => (int)$categoryId,
    'product_id'  => (int)$object->getId(),
    'position'    => 1
);

Replace 1 with 100.

Magento : Item (Mage_Catalog_Model_Product) with the same id already exist

We have fixed this issue in magento core file.

lib/Varien/Data/Collection.php

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;

   }