[{"_id":1,"items":["Test","Test","Test","Test","Test","Sword","Sword"]}]

db.collection.aggregate([{"$unwind":"$items"},{$group:{_id:"$items",cnt:{$sum:1},it:{$push:"$items"},origid:{"$first":"$_id"}}},{$project:{_id:"$origid",items:{$slice:["$it",0,2]}}},{$group:{_id:"$_id",items:{$push:"$items"}}},{$addFields:{items:{"$reduce":{"input":"$items","initialValue":[],"in":{"$concatArrays":["$$this","$$value"]}}}}}])