how to add same object in objectbox's box multiple times?

I have ToMany relation which looks like this,

@Entity()
class Pdf {
  @Id()
  int id;
  Uint8List pdfData;
  final String customerName;
  @Property(type: PropertyType.date)
  final DateTime dateTime;
  @Backlink()
  var products = ToMany<Product>();
  double totalAmount;
  PaymentStatus paymentStatus;

  Pdf({
    this.id = 0,
    required this.pdfData,
    required this.dateTime,
    required this.customerName,
    required this.totalAmount,
    this.paymentStatus = PaymentStatus.unPaid,
  });

  int get status => paymentStatus.index;

  set status(int value) {
    paymentStatus = PaymentStatus.values[value];
  }
}

@Entity()
class Product {
  @Id()
  int id;
  String name;
  String category;
  int categoryId;
  WeightType weightType;
  double? price;
  double weight;
  bool isRefunded;
  final pdf = ToOne<Pdf>();

  Product({
    this.id = 0,
    required this.name,
    required this.category,
    required this.categoryId,
    this.weightType = WeightType.kg,
    this.price,
    this.weight = 0,
    this.isRefunded = false,
  });
  Product copy() => Product(
        name: name,
        category: category,
        id: id,
        price: price,
        weight: weight,
        weightType: weightType,
        categoryId: categoryId,
        isRefunded: isRefunded,
      );
  }
@override
  int get hashCode => hashValues(
        name.hashCode,
        price.hashCode,
        id.hashCode,
        category.hashCode,
        categoryId.hashCode,
        weightType.hashCode,
        weight.hashCode,
        isRefunded.hashCode,
      );

  @override
  bool operator ==(Object other) =>
      other is Product &&
      other.name == name &&
      price == other.price &&
      category == other.category &&
      categoryId == other.categoryId &&
      other.weight == weight &&
      other.weightType == weightType &&
      other.id == id;
}

this is how I'm putting it in the box,

void add(){
final billPdf = Pdf(
      pdfData: pdf,
      dateTime: DateTime.now(),
      customerName: bill.customerName,
      totalAmount: bill.totalAmount,
      paymentStatus: bill.paymentStatus,
    );
    billPdf.products.addAll(bill.products);
    DBService.pdfBox.put(billPdf,mode: obj.PutMode.put);
}

now when I try add same product in the box, it removes the previous data from the list(db) and only adds the latest one so it is just updating the product. And if I change the PutMode to insert, it gives me exception. like this,

object put failed

I found this for kotlin, Can't save same data using ObjectBox with Android (Kotlin)

From this I even overrided the hashcode and put some different value but it still removes previous values and also in my use case everything can be same.

So How can I put same objects in object-box?

The problem was that I didn't need to add final pdf = ToOne<Pdf>(); in product's Model and remove the @Backlink() from Pdf model. And it put data in different list and doesn't remove from previous list. And if you put duplicate object it just updates it.

Although I still have one problem it only stores default values which means some values might be null but I think this problem is not related to question so it's fine and putting above as answer.