Simple example of using the messaging center.

1MasterPage.cs

 public ObservableCollection<TripExpense> Expenses { get; set; }

 public ExpensesViewModel()
 { 
   Expenses = new ObservableCollection<TripExpense>();

   //Subscibe to insert expenses
   MessagingCenter.Subscribe<TripExpense>(this, "AddNew", (expense) =>
   {
     Expenses.Add(expense);
   });

   //subscribe to update expenxes
   MessagingCenter.Subscribe<TripExpense>(this, "Update", (expense) =>
   {
     ExecuteUpdateExpense(expense);
   });
 }

2DetailViewModel.cs

private async Task ExecuteSaveCommand()
{
  if (IsBusy)
    return;

  IsBusy = true;
  
  //Send a message to insert/update the expense to all subscribers
  if(isNew)
  {
    MessagingCenter.Send(expense, "AddNew");
  }
  else
  {
    MessagingCenter.Send(expense, "Update");
  }
  IsBusy = false;
  navigation.PopAsync();
}

It works in page.cs side. but if i apply this in my view model, the list adds multiple items.
i dont know why it happens