Very strange behavior DIALOG

hello,

After close dialog thougth close button, sometimes it reopens inexplicably.

Dialog is open thougth message. It is reopened when the message is sent, however, only once.

public partial class AssociationDialog : Window
{
public AssociationDialog()
{
InitializeComponent();
}

private void Button_Click(object sender, RoutedEventArgs e)
{
this.DialogResult = true;
this.Close();
}
}

Messenger.Default.Register<NotificationMessage>(this, message =>
            {
                if (message.Notification == Notifications.AssociationDialog)
                {
                    var window = new AssociationDialog();
                    window.DataContext = new AssociationDialogViewModel();
                    window.ShowDialog();
                    if (window.DialogResult.HasValue && window.DialogResult.Value)
                    {
                        AssociationDialogViewModel vm = window.DataContext as AssociationDialogViewModel;
                        Messenger.Default.Send(new NotificationMessage<DictionaryEntry>(new DictionaryEntry(vm.Key, vm.Value), Notifications.AddAssociation));
                    }
                }
            });