[{"item":1313,"position":"2A","amount":500},{"item":1313,"position":"2A","amount":200},{"item":1414,"position":"1A","amount":500},{"item":1414,"position":"2A","amount":800},{"item":1313,"position":"1A","amount":300}]

db.collection.aggregate([{"$group":{"_id":{"item":"$item","positon":"$positon"},"items":{"$first":"$$ROOT"},"amount":{"$sum":"$amount"}}},{"$project":{"item":"$items.item","positon":"$items.position","amount":1,"_id":"$items._id"}}])