完成的异步下载会关闭两次而不是一次

我的应用程序中有这段代码，允许用户下载应用程序的最新版本。当应用程序下载完成后，如果用户希望打开文件位置以查看文件，则会打开提示符。

但是，该工具启动两个消息框，而不是只启动一次。我不确定我是否错过了什么。

private void BTN_GNV_MouseUp(object sender, MouseButtonEventArgs e)
{
    string URLDir = "http://shard.combatkingz.com/downloads/";
    string URLName = "DayZ Config Tweak tool v" + Properties.Settings.Default.AvailableVersion + ".exe";
    string URLFull = "";
    using (WebClient DLWC = new WebClient())
    {
        URLFull = URLDir + URLName;
        GlobalVars.DLPath = System.Environment.CurrentDirectory + "''" + URLName;
        try
        {
            DLWC.DownloadFileAsync(new Uri(URLFull), GlobalVars.DLPath);
            DLWC.DownloadProgressChanged += DLWC_DownloadProgressChanged;
        }
        catch
        {
            MessageBox.Show("There was an error downloading the file.", GlobalVars.APPNAME, MessageBoxButton.OK, MessageBoxImage.Error);
#if DEBUG
#else
            AddDownloadToDB("Failed");
#endif
        }
    }
}
void DLWC_DownloadProgressChanged(object sender, DownloadProgressChangedEventArgs e)
{
    PB_GNV.Width = (BTN_GNV.Width / 100) * e.ProgressPercentage;
    if (PB_GNV.Width == BTN_GNV.Width && e.TotalBytesToReceive == e.BytesReceived)
    {
        MessageBoxResult nav = MessageBox.Show("New version downloaded. Do you want to navigate to the folder?", GlobalVars.APPNAME, MessageBoxButton.YesNo, MessageBoxImage.Error);
        if (nav == MessageBoxResult.Yes)
        {
            string argument = @"/select, " + @GlobalVars.DLPath;
            System.Diagnostics.Process.Start("explorer.exe", argument);
#if DEBUG
#else
            AddDownloadToDB("Success");
#endif
        }
    }
}