[{"_id":"5a760191813a54000b8475f1","orders":[{"row":"3","seat":"11"},{"row":"3","seat":"12"}],"product_id":"5a7628bedbcc42000aa7f614"},{"_id":"5a75f6f17abe45000a3ba05e","orders":[{"row":"3","seat":"12"},{"row":"3","seat":"11"}],"product_id":"5a7628bedbcc42000aa7f614"},{"_id":"5a75ebdf813a54000b8475e7","orders":[{"row":"5","seat":"16"},{"row":"5","seat":"15"}],"product_id":"5a75f711dbcc42000c459efc"},{"_id":"5a75ebdf813a54000b8475g7","orders":[{"row":"5","seat":"155"},{"row":"1","seat":"15"}],"product_id":"5a75f711dbcc42000c459efc"}]

db.collection.aggregate([{"$unwind":"$orders"},{$sort:{"orders.row":1,"orders.seat":1}},{$group:{_id:"$_id",tmpOrders:{$push:{$concat:["$orders.row","$orders.seat"]}},product_id:{$first:"$product_id"}}},{$group:{_id:{orders:"$tmpOrders",product:"$product_id"},dupIds:{$push:"$_id"}}},{$match:{"dupIds.1":{$exists:true}}},{$project:{_id:0,dups:"$dupIds",}}])