[{"_id":1,"sender":"John","receiver":"Kate","description":"test1"},{"_id":2,"sender":"John","receiver":"Kate","description":"test2"},{"_id":3,"sender":"Mike","receiver":"Kate","description":"test3"}]

db.collection.aggregate([{$group:{_id:{"sender":"$sender"},r:{"$push":"$$ROOT"}}},{"$project":{sender:"$_id.sender",receiver:{$first:"$r.receiver"},description:{$first:"$r.description"},_id:0}}])