WindowOpened Event Fires Twice

<ribbon:RibbonWindow 
    x:Class="TestApp.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:ribbon="http://schemas.actiprosoftware.com/winfx/xaml/ribbon"
    xmlns:themes="http://schemas.actiprosoftware.com/winfx/xaml/themes"
    xmlns:docking="clr-namespace:ActiproSoftware.Windows.Controls.Docking;assembly=ActiproSoftware.Docking.Wpf351"
    Title="Window1" 
    Height="300" 
    Width="300"
    themes:ThemeManager.Theme="LunaHomestead"
    >
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto" />
            <RowDefinition Height="*" />
        </Grid.RowDefinitions>
        <Button Content="Add Window" Click="Button_Click" />
        <docking:DockSite x:Name="uxDockSite_DockSite" Grid.Row="1" WindowOpened="uxDockSite_DockSite_WindowOpened">
            <docking:Workspace>
                <docking:TabbedMdiHost>
                    <docking:TabbedMdiContainer>
                        <docking:DocumentWindow Title="Hello World" />
                    </docking:TabbedMdiContainer>
                </docking:TabbedMdiHost>
            </docking:Workspace>
        </docking:DockSite>
    </Grid>
</ribbon:RibbonWindow>

using System;
using System.Windows;
using ActiproSoftware.Windows.Controls.Docking;

namespace TestApp
{
    /// <summary>
    /// Interaction logic for Window1.xaml
    /// </summary>
    public partial class Window1
    {
        public Window1()
        {
            InitializeComponent();
        }

        private void uxDockSite_DockSite_WindowOpened(object sender, ActiproSoftware.Windows.Controls.Docking.DockingWindowEventArgs e)
        {
            Console.WriteLine("Window Opened");
        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            DocumentWindow window = new DocumentWindow(uxDockSite_DockSite) {Title = "Another Window"};
            window.Activate();
        }
    }
}