Function ignoring condition and executing more than once

Question

I'm trying to create a function that adds an item into a table when it doesn't exist yet, but instead of adding the item once and stop executing, it adds the item more than once, even if it's already added. I tried this way:

List<Meal> _items = [];
  List<Meal> get items => [..._items];

 @override
  void initState() {
super.initState();
    loadMeals();
  }

Future<void> loadMeals() async {
    _loggedItems.clear();
    // _items.clear();
    String? teste = user?.email.toString();
    final responseUser = await http.get(
      Uri.parse(
          'URL/users_login/$teste'), // ${_formData.email}
      headers: <String, String>{
        'Content-Type': 'application/json; charset=UTF-8',
      },
    );
    if (responseUser.body == 'null') return;
    final data2 = jsonDecode(responseUser.body)['items'] as List?;

    // print(teste2);

    if (data2 != null) {
      _loggedItems = data2.map((e) => UserData.fromMap(e)).toList();
    }

    final response = await http.get(
      Uri.parse(
          'URL/pratos/${dropDownService.replaceAll('à', 'a')}/${loggedItems.first.restaurantId}'),
      headers: <String, String>{
        'Content-Type': 'application/json; charset=UTF-8',
        'Accept': 'application/json; charset=UTF-8'
      },
    );
    if (response.body == 'null') return;
    final data = jsonDecode(utf8.decode(response.bodyBytes))['items'] as List?;

    if (data != null) {
      _items = data.map((e) => Meal.fromMap(e)).toList();
    }
    setState(() {});
  }

void _menuWithReservation() {
    final loggedUserList = Provider.of<LoggedUser>(context, listen: false);
    final newItem = MenuMealRestaurant(
      menuId: '161',
      mealId: (dropDownService == 'gastronomia') ? '6' : '65',
      restaurantId: loggedUserList.loggedItems.first.restaurantId,
    );
    print('chamou');

    addMealOnMenu(newItem);
  }

  Future<void> addMealOnMenu(MenuMealRestaurant menuMealRestaurant) async {
    // ignore: unused_local_variable
    final future = await http.post(
      Uri.parse(
          'URL/cardapio_prato_restaurante_post'),
      headers: <String, String>{
        'Content-Type': 'application/json; charset=UTF-8',
      },
      body: jsonEncode(<String, String>{
        "id_cardapio": menuMealRestaurant.menuId.toString(),
        "id_prato": menuMealRestaurant.mealId.toString(),
        "id_restaurante": menuMealRestaurant.restaurantId.toString(),
      }),
    );
    _itemsMealAdd.add(menuMealRestaurant);
    //  return future.then((response) {

    //   });
  }

 @override
  Widget build(BuildContext context) {
    var index = daysMap[dropDownDayOfWeek];
    var date = calendar(int.parse(index!));
    final reservationButtonExists = _items
        .where((element) =>
            element.mealName.contains('Reservar') &&
            DateFormat('dd-MM-yyyy').format(DateTime.parse(element.date!)) ==
                date)
        .toList();
    if (dropDownService != 'sabor à mesa' &&
        _option == Option.reserva &&
        reservationButtonExists.isEmpty) {
      _menuWithReservation();
    }
}

It should only execute the function "_menuWithReservation" while reservationButtonExists is empty, but instead it executes a few couple times before it starts to respect the condition. How can I fix this?