[{name:"Bob",age:"22"},{name:"John",age:"21"},{name:"Pedro",age:"15"},{name:"Bob",age:"11"},{name:"Mark",age:"24"}]

db.collection.aggregate([{$group:{_id:"$name",count:{$sum:1}}},{$match:{count:1}},{$group:{_id:null,unique:{$push:"$_id"}}},{$project:{_id:0}}])