Merging duplicate models in my collection but adding the quantity

Question

I have items that are stored as models through an add to cart button. The model has:

• Name
• Quantity
• Item_ID

But it will not store duplicate items. If someone clicks on the add to cart button again I want the collection to merge the 2 models together by adding the quantities together but keeping the name/item_id the same.

I'd imagine I need to override the collection add function, to do specifically this when it sees a duplicate. Or is there some kind of method or parameter I can use when adding to my collection to do this for me?

var CartCollection = Backbone.Collection.extend({                   
    model: Item,
    add: function(newItem) {
      var dupeHandle = this.any(function(item) {
              if (item.get('name') === newItem.get('name')){
                  item.set('quantity', (item.get('quantity') + newItem.get('quantity'));
                  return true;
              }
      }
      if (dupeHandle) return;
      Backbone.Collection.add.call(this, item);
    } 
});