[{"user_id":1,"user_name":"george"},{"user_id":2,"user_name":"bill"},{"user_id":3,"user_name":"donny"},{"user_id":1,"user_name":"george"},]

db.collection.aggregate([{$group:{_id:{user_id:"$user_id",user_name:"$user_name"},}},{$project:{_id:0,user_id:"$_id.user_id",user_name:"$_id.user_name"}}])