Silverlight：Element已经是使用StackPanel，ObservableCollection的另一个元素的子元素

Question

我有一些XAML：

<StackPanel Grid.Row="1" x:Name="avlLegend" Orientation="Horizontal" >
        <sdk:Label Grid.Row="1" Height="20" HorizontalAlignment="Left" Foreground="White" Name="lblLegend" Content="Legend:" FontWeight="Black" VerticalAlignment="Center" Width="57" Margin="12,8,0,11" />
        <ItemsControl Grid.Row="1" Name="avlLegendItems" ItemsSource="{Binding Path=AVLAlertLegend}"  >
            <ItemsControl.ItemsPanel>
                <ItemsPanelTemplate>
                    <StackPanel Orientation="Horizontal" />
                </ItemsPanelTemplate>
            </ItemsControl.ItemsPanel>
        </ItemsControl>
    </StackPanel>

我用它来绑定：

private ObservableCollection<AVLAlert> _AVLAlertLegendCollection = new ObservableCollection<AVLAlert>();          

    public ObservableCollection<AVLAlert> AVLAlertLegendCollection
    {
        get { return _AVLAlertLegendCollection; }
        set
        {
            _AVLAlertLegendCollection = value;
            RaisePropertyChanged("AVLAlertLegendCollection");
        }
    }

    public ObservableCollection<FrameworkElement> AVLAlertLegend
    {
        get { return _AVLAlertLegend; }
        set
        {
            _AVLAlertLegend = value;
            RaisePropertyChanged("AVLAlertLegend");
        }
    }

    public void LoadAvlLegend()
    {
        foreach (AVLAlert avl in AVLAlertLegendCollection)
        {
            Image img = new Image();
            img.Source = new System.Windows.Media.Imaging.BitmapImage(new Uri(String.Format(@"/{0}.png",avl.AlertIconColor), UriKind.RelativeOrAbsolute));
            img.Height = 25;
            img.Width = 25;
            img.Margin = new Thickness(15, 0, 0, 0);

            DropShadowEffect dropShadow = new DropShadowEffect();
            dropShadow.BlurRadius = 3;
            dropShadow.Color = Colors.White;
            dropShadow.Direction = 0;
            dropShadow.Opacity = 1.0;
            dropShadow.ShadowDepth = 0;

            img.Effect = dropShadow;

            AVLAlertLegend.Add(img);

            Label lbl = new Label();
            lbl.Content = avl.Description;
            lbl.Foreground = new SolidColorBrush(Colors.White);
            lbl.Margin = new Thickness(10, 0, 0, 0);
            AVLAlertLegend.Add(lbl);
        }

    }

首次进入视图时，它会加载正常。但是当我离开并回到视野时，我得到的元素已经是另一个元素的孩子了。错误。我做错了什么？