[{"repo_id":667006,user_id:1060},{"repo_id":667006,user_id:1060},{"repo_id":667006,user_id:2467194},{"repo_id":667006,user_id:21979}]

db.collection.aggregate([{$group:{_id:{repo_id:"$repo_id",user_id:"$user_id"},doc:{$first:"$$ROOT"}}},{$replaceRoot:{"newRoot":"$doc"}}])