[{"type":"one"},{"type":"two"},{"type":"one"},{"type":"three"},{"type":"one"},{"type":"three"}]

db.collection.aggregate([{"$group":{"_id":"$type","docs":{"$push":"$$ROOT"}}},{"$project":{docs:{"$slice":["$docs",2]},_id:0,},},{"$unwind":"$docs"},{"$replaceRoot":{"newRoot":"$docs"}}])