DocumentWindow IsVisibleChanged fires multiple times

<Window
    x:Class="DocumentWindowIsVisibleChanged.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:docking="http://schemas.actiprosoftware.com/winfx/xaml/docking"
    Title="MainWindow" Height="350" Width="525">
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition />
            <RowDefinition Height="auto" />
        </Grid.RowDefinitions>
        <docking:DockSite Name="docksite">
            <docking:Workspace>
                <docking:TabbedMdiHost>
                    <docking:TabbedMdiContainer />
                </docking:TabbedMdiHost>
            </docking:Workspace>
        </docking:DockSite>
        <Button Grid.Row="1" Content="Create new window and dock" Click="Button_Click" />
    </Grid>
</Window>

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

namespace DocumentWindowIsVisibleChanged
{
    public partial class MainWindow : Window
    {
        int windowNumber = 0;
        private static string calls = string.Empty;
        public MainWindow() { InitializeComponent(); }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            calls = string.Empty;
            DocumentWindow dw = new DocumentWindow(docksite, "window" + windowNumber, "window" + windowNumber, null, null) { Tag = "window" + windowNumber };
            dw.IsVisibleChanged += dw_IsVisibleChanged;

            dw.Activate();
            MessageBox.Show(calls, "DocumentWindow.IsVisibleChanged call History");

            windowNumber++;
        }

        private void dw_IsVisibleChanged(object sender, DependencyPropertyChangedEventArgs e)
        {
            calls += string.Format("{0}: OldValue={1}, NewValue={2}{3}", (string)((DocumentWindow)sender).Tag, e.OldValue, e.NewValue, Environment.NewLine);
        }
    }
}