[{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([{$match:{}},{$group:{_id:{item:"$item",position:"$position"},total:{$sum:"$amount"}}},{$project:{_id:0,item:"$_id.item",position:"$_id.position",total:1}}])