Making a new list from existing list in flutter; changes both list upon action on new list

i am making a list in flutter from an existing list and when i used this new list and perform some action on it like change its quantity parameter it changes the parameter of existing list also. Can some one tell me where i am going wrong.

Existing list:

  List<ItemsModel> originalList=[
    ItemsModel(name: 'High Quality Double Spring Tummy Trimmer For Unisex',quantity: '30'),
    ItemsModel(name: 'High Quality Double Spring Tummy Trimmer For Unisex',quantity: '30'),
    ItemsModel(name: 'High Quality Double Spring Tummy Trimmer For Unisex',quantity: '30'),
  ];

New list:

  List<ItemsModel> updatedItemList;
      void initState() {
        updatedItemList=[];
        originalList.forEach((val){
      updatedItemList.add(val);
    });
        super.initState();
      }

on pressed button:

 updatedItemList[i].quantity=qtyReceive

And on some button pressed i am performing this action that was updating both lists. Why??

As @jamesdlin mentioned, creating a deep copy will solve the issue by change reference to new ItemsModel objects in the list.

Small example of creating deep copy:

class ItemsModel{
  String name;
  String quantity;
  ItemsModel({this.name,this.quantity});
}


void main() {
   List<ItemsModel> originalList=[
    ItemsModel(name: 'High Quality Double Spring Tummy Trimmer For Unisex',quantity: '30'),
    ItemsModel(name: 'High Quality Double Spring Tummy Trimmer For Unisex',quantity: '30'),
    ItemsModel(name: 'High Quality Double Spring Tummy Trimmer For Unisex',quantity: '30'),
  ];

  List<ItemsModel> updatedItemList;
  updatedItemList=[];
  originalList.forEach((val){
      //creating new ItemsModel objects and adding to the updatedList
      updatedItemList.add(ItemsModel(name:val.name,quantity:val.quantity));
    });

  print('Original List');
  for(var listItem in originalList){
    listItem.quantity = '60';
    print('${listItem.name},${listItem.quantity}');
  }

  print('Updated List');
    for(var listItem in updatedItemList){
    print('${listItem.name},${listItem.quantity}');
  }
}