Data Grid Error when Reloading page

I have an Add Customer UI which has a data grid applied to show the entries provided, but if I re select Add Customer while already showing it will add the entries again.

rows 1-3 are the original and 4-6 are duplicates. Does anyone know how to limit this from happening??

in my MainActivity i have a menu navigation with a Frame to populate the xaml pages. And below is the code to initialize the class.

private void AddCustomer_Click(object sender, RoutedEventArgs e)
{
    Main.Content = new AddCustomerUI();
}

The Frame is called Main and then assigning the xaml page to its Main.content

public partial class AddCustomerUI : Page
{
    public AddCustomerUI()
    {
        InitializeComponent();
        SetupCustomer();
        customerDataGrid.ItemsSource = App.customers;
    }
    private void SetupCustomer()
    {
        var customer = new Customer { FirstName = "Timothy", LastName = "Jennings", Email = "tim.jennings@gmail.com", Phone = "0275 202020",
            Address = new Address { Street = "100 Burt Road", Suburb = "Howick", City = "Auckland", Country = "New Zealand" }
        };
        App.customers.Add(customer);
        customer = new Customer { FirstName = "Brian", LastName = "Jones", Email = "bjones@gmail.com", Phone = "0275 903070",
            Address = new Address { Street = "100 Vincent Road", Suburb = "St Lukes", City = "Auckland", Country = "New Zealand" }
        };
        App.customers.Add(customer);
        App.customers.Add(new Customer { FirstName = "Terry", LastName = "Teo", Email = "tete@mana.com", Phone = "021 756 382",
            Address = new Address { Street = "23 Ford St", City = "Auckland", Suburb = "Pakuranga", Country = "New Zealand" }
        });

    }

    private void SaveCustomer_Click(object sender, RoutedEventArgs e)
    {
        if (string.IsNullOrWhiteSpace(fNameTxt.Text) || string.IsNullOrWhiteSpace(lNameTxt.Text) || string.IsNullOrWhiteSpace(emailTxt.Text)
            || string.IsNullOrWhiteSpace(phTxt.Text))
            //MessageBox.Show("Empty Fields", "Please look at filling out all fields on the form.");
            return;
        var customer = new Customer
        {
            FirstName = fNameTxt.Text, LastName = lNameTxt.Text, Email = emailTxt.Text, Phone = phTxt.Text,
            Address = new Address { Street = streetTxt.Text, Suburb = suburbTxt.Text, City = cityTxt.Text, Country = countryTxt.Text }
        };
        App.customers.Add(customer);
        customerDataGrid.Items.Refresh();
        ResetText();
    }
    private void ResetText()
    {
        fNameTxt.Text = string.Empty; lNameTxt.Text = string.Empty;
        emailTxt.Text = string.Empty; phTxt.Text = string.Empty;
        streetTxt.Text = string.Empty; cityTxt.Text = string.Empty;
        suburbTxt.Text = string.Empty; countryTxt.Text = string.Empty;
    }
}

Solution

The issue that you have is in SetupCustomer, you're not clearling the list so you have multiple entries in your collection.

private void SetupCustomer()
{
    App.customers.Clear();
    var customer = new Customer { FirstName = "Timothy", LastName = "Jennings", Email = "tim.jennings@gmail.com", Phone = "0275 202020",
     Address = new Address { Street = "100 Burt Road", Suburb = "Howick", City = "Auckland", Country = "New Zealand" }
        };
     App.customers.Add(customer);
     customer = new Customer { FirstName = "Brian", LastName = "Jones", Email = "bjones@gmail.com", Phone = "0275 903070",
     Address = new Address { Street = "100 Vincent Road", Suburb = "St Lukes", City = "Auckland", Country = "New Zealand" }
        };
     App.customers.Add(customer);
     App.customers.Add(new Customer { FirstName = "Terry", LastName = "Teo", Email = "tete@mana.com", Phone = "021 756 382",
     Address = new Address { Street = "23 Ford St", City = "Auckland", Suburb = "Pakuranga", Country = "New Zealand" }
     });

}