[{"items":[{"item_id":1},{"item_id":2},{"item_id":3},]},{"items":[{"item_id":1},{"item_id":2},{"item_id":1},]}]

db.collection.aggregate([{$match:{$expr:{$ne:[{$size:"$items"},{$size:{$reduce:{input:"$items",initialValue:[],in:{$setUnion:["$$value",["$$this.item_id"]]}}}}]}}}])