[{"po_no":"456","amount":0.1},{"po_no":"455","amount":0.2},{"po_no":"454","amount":0.3},{"po_no":"456","amount":0.4}]

db.collection.aggregate([{$group:{_id:"$po_no",amount:{$push:"$amount"}}},{$match:{$expr:{$gt:[{$size:"$amount"},1]}}},{$unwind:"$amount"},{$project:{_id:0,po_no:"$_id",amount:1}}])