[{"_id":1,"name":"Sally","age":18},{"_id":2,"name":"Bob","age":18},{"_id":3,"name":"Bob","age":18}]

db.collection.aggregate([{$match:{name:{$in:["Bob","Sally"]}}},{$group:{"_id":"$name",fields:{$push:{name:"$name",age:"$age"}}}},{$sort:{_id:-1}},{$replaceRoot:{newRoot:{$arrayToObject:[[{k:"$_id",v:"$fields"}]]}}}])