[{"item_code":"00001","description":"description for item 1",},{"item_code":"00002","description":"description for item 2",},{"item_code":"00003","description":"description for item 3",},{"item_code":"00002","description":"asdasda","extrafield":"extra field for item 2",}]

db.collection.aggregate([{$group:{_id:"$item_code",root:{$mergeObjects:"$$ROOT"}}},{$replaceWith:"$root"}])