Magento Error: Item (Mage_Core_Model_Store) with the same id “0” already exist
There can be multiple rows inserted with the same value in tables core_store, core_website and core_store_group.
You have to delete such duplicate rows from core_store, core_website and core_store_group table. That is, if you have three rows with same value then remove the two duplicate rows.
Refresh Cache from System -> Cache Management.