Wpf 用户控件Load事件执行两次

<!--主窗体，放置一个用户控件-->
<Window x:Class="WpfApplication.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" xmlns:wpfApplication="clr-namespace:WpfApplication"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <wpfApplication:UserControl1></wpfApplication:UserControl1>
    </Grid>
</Window>

//用户控件
namespace WpfApplication
{
    /// <summary>
    /// UserControl1.xaml 的交互逻辑
    /// </summary>
    public partial class UserControl1 : UserControl
    {
        public UserControl1()
        {
            InitializeComponent();
        }

        private void UserControl1_OnLoaded(object sender, RoutedEventArgs e)
        {
　　　　　　　//为毛执行两次？
            StackTrace trace = new StackTrace();
            MessageBox.Show(trace.ToString());
        }
    }
}