[{id:"some value","product/productId":"some value","product/title":"some value","product/price":"unknown"},{id:"some value","product/productId":"some value","product/title":"some value","product/price":"12.57"},{id:"only one","product/productId":"some value","product/title":"some value","product/price":"12.57"}]

db.collection.aggregate([{"$group":{"_id":"$id","price":{"$push":"$product/price"}}},{"$match":{"$expr":{"$gt":[{"$size":"$price"},1]}}}])