Viewを内部に持つViewを複数生成できない

Viewを内部に持つViewを複数生成したいです。

ListBoxの数値を選択すると、選択された数値を受け取ったViewを表示するプログラムを作っています。
以前表示したViewは残して、異なる数値が選択された場合、新しくViewを生成するようにしたいです。

Viewの内部にViewを持っていると1→2に変えたときに下記のエラーが出ます。

ArgumentException: Region with the given name is already registered: ViewBRegion

WPF
.NET Core 3
Prism.Unity(7.2.0.1367)
ReactiveProperty(6.1.4)

<フォルダ構成>

ViewModelsフォルダ

• MainWindowViewModel.cs
• ViewAViewModel.cs
• ViewBViewModel.cs

Viewsフォルダ

• UserControls/ViewA.xaml
• UserControls/ViewB.xaml
• MainWindow.xaml

App.xaml

MainWindowViewModel.cs

using Prism.Mvvm;
using Prism.Regions;
using Reactive.Bindings;
using System;
using System.Collections.Generic;
using Test.Views.UserControls;
using Unity;

namespace Test.ViewModels
{
    public class MainWindowViewModel : BindableBase
    {
        [Dependency]
        public IRegionManager RegionManager { get; set; }
        public List<int> NumberList { get; private set; }
        public ReactiveProperty<int> SelectedNumber { get; set; }
        public MainWindowViewModel()
        {
            this.NumberList = new List<int> { 1, 2, 3 };
            this.SelectedNumber = new ReactiveProperty<int>();

            this.SelectedNumber.Subscribe(_=>this.ChangeView()) ;
        }

        private void ChangeView()
        {
            if(RegionManager != null)
            {
                this.RegionManager.RequestNavigate("ContentRegion", nameof(ViewA), new NavigationParameters($"id={this.SelectedNumber.Value}"));
            }
        }
    }
}

ViewAViewModel.cs

using Prism.Commands;
using Prism.Mvvm;
using Prism.Regions;
using Reactive.Bindings;
using System;
using System.Collections.Generic;
using System.Linq;

namespace Test.ViewModels
{
    public class ViewAViewModel : BindableBase, INavigationAware
    {
        public ReactiveProperty<string> Id { get; private set; }
        public ViewAViewModel()
        {
            Id = new ReactiveProperty<string>();
        }

        public bool IsNavigationTarget(NavigationContext navigationContext)
        {
            return this.Id.Value == navigationContext.Parameters["id"] as string;
        }

        public void OnNavigatedFrom(NavigationContext navigationContext)
        {

        }

        public void OnNavigatedTo(NavigationContext navigationContext)
        {
            this.Id.Value = navigationContext.Parameters["id"] as string;
        }
    }
}

ViewBViewModel.cs

using Prism.Commands;
using Prism.Mvvm;
using System;
using System.Collections.Generic;
using System.Linq;

namespace Test.ViewModels
{
    public class ViewBViewModel : BindableBase
    {
        public ViewBViewModel()
        {

        }
    }
}

MainWindow.xaml

<Window x:Class="Test.Views.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:prism="http://prismlibrary.com/"
        prism:ViewModelLocator.AutoWireViewModel="True"
        Title="{Binding Title}" Height="350" Width="525" >
    <StackPanel>
        <ListBox ItemsSource="{Binding NumberList}" SelectedItem="{Binding SelectedNumber.Value}">
            <ListBox.ItemTemplate>
                <DataTemplate>
                    <TextBlock Text="{Binding}"/>
                </DataTemplate>
            </ListBox.ItemTemplate>
            <ListBox.ItemsPanel>
                <ItemsPanelTemplate>
                    <StackPanel Orientation="Horizontal"/>
                </ItemsPanelTemplate>
            </ListBox.ItemsPanel>
        </ListBox>
        <ContentControl prism:RegionManager.RegionName="ContentRegion" />
    </StackPanel>
</Window>

ViewA.xaml

<UserControl x:Class="Test.Views.UserControls.ViewA"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:prism="http://prismlibrary.com/"             
             prism:ViewModelLocator.AutoWireViewModel="True"
             Loaded="UserControl_Loaded">
    <StackPanel>
        <TextBlock Text="{Binding Id.Value}"/>
        <ContentControl prism:RegionManager.RegionName="ViewBRegion" />
    </StackPanel>
</UserControl>

ViewA.xaml.cs

using Prism.Ioc;
using Prism.Regions;
using System.Windows.Controls;
using Unity;

namespace Test.Views.UserControls
{
    /// <summary>
    /// Interaction logic for ViewA
    /// </summary>
    public partial class ViewA : UserControl
    {
        [Dependency]
        public IContainerExtension ContainerExtension { get; set; }
        [Dependency]
        public IRegionManager RegionManager { get; set; }
        public ViewA()
        {
            InitializeComponent();
        }

        private void UserControl_Loaded(object sender, System.Windows.RoutedEventArgs e)
        {
            RegionManager.AddToRegion("ViewBRegion", ContainerExtension.Resolve<ViewB>());

        }
    }
}

ViewB.xaml

<UserControl x:Class="Test.Views.UserControls.ViewB"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:prism="http://prismlibrary.com/"             
             prism:ViewModelLocator.AutoWireViewModel="True">
    <Grid>
        <TextBlock Text="ViewB"/>
    </Grid>
</UserControl>

app.xaml.cs

using Prism.Ioc;
using Test.Views;
using System.Windows;
using Test.Views.UserControls;
using Prism.Mvvm;
using Test.ViewModels;

namespace Test
{
    /// <summary>
    /// Interaction logic for App.xaml
    /// </summary>
    public partial class App
    {
        protected override Window CreateShell()
        {
            return Container.Resolve<MainWindow>();
        }

        protected override void RegisterTypes(IContainerRegistry containerRegistry)
        {
            containerRegistry.RegisterForNavigation<ViewA>();
        }
        protected override void ConfigureViewModelLocator()
        {
            base.ConfigureViewModelLocator();
            ViewModelLocationProvider.Register<ViewA, ViewAViewModel>();
            ViewModelLocationProvider.Register<ViewB, ViewBViewModel>();

        }
    }
}