[{"_id":"5a760191813a54000b8475f1","orders":[{"row":"3","seat":"11"},{"row":"3","seat":"12"}],"product_id":"5a7628bedbcc42000aa7f614"},{"_id":"5a75f6f17abe45000a3ba05e","orders":[{"row":"3","seat":"12"},{"row":"3","seat":"13"}],"product_id":"5a7628bedbcc42000aa7f614"},{"_id":"5a75ebdf813a54000b8475e7","orders":[{"row":"5","seat":"16"},{"row":"5","seat":"15"}],"product_id":"5a75f711dbcc42000c459efc"}]

db.collection.aggregate([{"$unwind":"$orders"},{$group:{"_id":{"order":"$orders","product_id":"$product_id"},"count":{$sum:1},"doc_ids":{$push:"$_id"}}},{$match:{count:{$gt:1}}},{"$lookup":{"from":"collection","localField":"doc_ids","foreignField":"_id","as":"documents"}},{"$unwind":"$documents"},{"$replaceRoot":{"newRoot":"$documents"}}])