How to find WPF page in navigation history to prevent it from being opened again

Pages are rarely used in commercial applications. As far as I understand they are indeed for xbap (wpf in browser). It is more common to host user controls in contentcontrols. In this case, there are two common options

1) Look first. Holds a dictionary of user controls typed by type. https://gallery.technet.microsoft.com/WPF-Navigation-Basic-Sample-11f10c74

    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
            Navigate(typeof(HomeView));
        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            Button btn = (Button)e.OriginalSource;
            Navigate(btn.Tag as Type);
        }

        private void Navigate (Type viewType)
        {
            UserControl uc;
            if (Views.ContainsKey(viewType))
            {
                uc = Views[viewType];
            }
            else
            {
                uc = (UserControl)Activator.CreateInstance(viewType);
                Views[viewType] = uc;
            }
            NavigationParent.Content = uc;
        }
        private Dictionary<Type, UserControl> Views = new Dictionary<Type, UserControl>();
    }

Proponents point out that navigation can be said to be a view responsibility.

2) First use Viewmodel.

Bind the content of the contentcontrol to a property in your mainwindowviewmodel. Switch it to the view model of each view, and switch it to the user control. You can then control the retention state by keeping a reference to the view model for each view.

If you really want to stick to what you have, then Frame.BackStack is a collection of entries in a journal. You can iterate over it and check the type of each object. I think it's actually a reference to the page. I've never seen this approach used in a commercial application.