How to get an ItemsSource to refresh its bind?

I've got a view which shows a listbox that is bound to GetAll():

<DockPanel>
    <ListBox ItemsSource="{Binding GetAll}"
             ItemTemplate="{StaticResource allCustomersDataTemplate}"
             Style="{StaticResource allCustomersListBox}">
    </ListBox>
</DockPanel>

GetAll() is an ObservableCollection property in my ViewModel:

public ObservableCollection<Customer> GetAll
{
    get
    {
        return Customer.GetAll();
    }
}

which in turn calls a GetAll() model method which reads an XML file to fill the ObservableCollection.:

public static ObservableCollection<Customer> GetAll()
{
    ObservableCollection<Customer> customers = new ObservableCollection<Customer>();

    XDocument xmlDoc = XDocument.Load(Customer.GetXmlFilePathAndFileName());
    var customerObjects = from customer in xmlDoc.Descendants("customer")
                          select new Customer
                          {
                              Id = (int)customer.Element("id"),
                              FirstName = customer.Element("firstName").Value,
                              LastName = customer.Element("lastName").Value,
                              Age = (int)customer.Element("age")
                          };
    foreach (var customerObject in customerObjects)
    {
        Customer customer = new Customer();

        customer.Id = customerObject.Id;
        customer.FirstName = customerObject.FirstName;
        customer.LastName = customerObject.LastName;
        customer.Age = customerObject.Age;

        customers.Add(customer);
    }

    return customers;
}

This all works fine EXCEPT when the user goes to another view, edits the XML file and comes back to this view where the old data is still showing.

Ho