[{"_id":"5f37dad305c1b9403bfe808b","nextBilling":{"isAutomatic":true,"isRedeemedForFutureBilling":false,"time":1000,"duration":10000000},"userToken":"aaaa","__v":0},{"_id":"5f37dad305c1b9403bfe323b","nextBilling":{"isAutomatic":true,"isRedeemedForFutureBilling":false,"time":20000000000000,"duration":1000},"userToken":"aaaa","__v":0},{"_id":"5f37dfj1b9403bfe323b","nextBilling":{"isAutomatic":true,"isRedeemedForFutureBilling":false,"time":1000,"duration":1000},"userToken":"bbbb","__v":0}]

db.collection.aggregate([{$sort:{"nextBilling.time":-1}},{$group:{_id:"$userToken",id:{$first:"$_id"},nextBilling:{$first:"$nextBilling"}}},{$project:{_id:"$id",userToken:"$_id",nextBilling:1}}])