[{_id:ObjectId("5cd9010310b80b3e38cd3f88"),subGroup:[{bookList:[{title:"A good book",id:"abc123"}]}]},{_id:ObjectId("5cd9010710b80b3e38cd3f89"),subGroup:[{bookList:[{title:"A good book",id:"abc123"}]}]}]

db.collection.aggregate([{"$unwind":"$subGroup"},{"$unwind":"$subGroup.bookList"},{"$group":{"_id":"$subGroup.bookList.title","count":{"$sum":1}}},{"$match":{"_id":{"$ne":null},"count":{"$gt":1}}}])