UWP prevent file activation from overriding previous frame

Question

I'm trying to make a tab appear for every file opened via File Explorer.

Expectations: upon double click on the file, it should open up a new tab for that file

Reality: every time I open a file, the frame gets overwritten.

MainPage - contains tabs

TabbedMainPage - the workspace

App.cs:

    protected override void OnFileActivated(FileActivatedEventArgs Args)
    {
        base.OnFileActivated(Args);
        Frame RF = Window.Current.Content as Frame;
        if (RF != null)
        {
            var Y = new FrameNavigationOptions();
            Y.IsNavigationStackEnabled = false;
            RF.NavigateToType(typeof(MainPage), Args, Y);
        }
        else
        {
            RF = new Frame();
            Window.Current.Content = RF;
            RF.Navigate(typeof(MainPage), Args);
            Window.Current.Activate();
        }
    }

MainPage.cs:

    protected override void OnNavigatedTo(NavigationEventArgs EvArgs)
    {
        //Catch file
        base.OnNavigatedTo(EvArgs);
        var Args = EvArgs.Parameter as IActivatedEventArgs;
        var FArgs = Args as FileActivatedEventArgs;
        if (Args != null && Args.Kind == ActivationKind.File)
        {
            //Write file properties
            TabViewItem NewTabItem = new TabViewItem();
            NewTabItem.Header = "New Tab";
            NewTabItem.IconSource = new Microsoft.UI.Xaml.Controls.SymbolIconSource() { Symbol = Symbol.Document };
            Frame RF = new Frame();
            RF.Navigate(typeof(TabbedMainPage), Args);
            NewTabItem.Content = RF;
            TabbedView.TabItems.Add(NewTabItem);
            TabbedView.UpdateLayout();
        }
        else { }
    }

TabbedMainPage.cs:

    protected override async void OnNavigatedTo(NavigationEventArgs EvArgs)
    {
        //Catch file
        base.OnNavigatedTo(EvArgs);
        var Args = EvArgs.Parameter as IActivatedEventArgs;
        var FArgs = Args as FileActivatedEventArgs;
        if (Args != null && Args.Kind == ActivationKind.File)
        {
            //Write file properties
            TXTFile = FArgs.Files[0] as StorageFile;
            var Str = await TXTFile.OpenReadAsync();
            //Read file
            REB.Document.LoadFromStream(TextSetOptions.FormatRtf, Str);
            RTB.Document.LoadFromStream(TextSetOptions.FormatRtf, Str);
            Str.Dispose();
            HomePage.Visibility = Visibility.Collapsed;
            CheckForSaving();
        }
        else { }
    }