WinForms有时出现两次，有时出现一次

我对此感到困惑，名为"联系人"的windows窗体有时在《欢迎屏幕》之后调用两次，有时它（"联系人"）只调用一次。我确信我只调用过一次表单。

这是我正在使用的代码：

"WelcomeScreen"下面的表单是运行程序时调用的第一个表单：

public partial class WelcomeScreen : Form
    {
        int timeLeft = 5;
        Timer _timer = new Timer();
        BackgroundWorker _backgroundWorker = new BackgroundWorker();
        public WelcomeScreen()
        {
            InitializeComponent();
            _backgroundWorker.WorkerReportsProgress = true;
            _backgroundWorker.DoWork += BackgroundWorker_DoWork;
            _backgroundWorker.ProgressChanged += BackgroundWorker_ProgressChanged;
            _timer.Interval = 1000;
            _timer.Tick += Timer_Tick;
        }
        void WelcomeScreen_Load(object sender, EventArgs e)
        {
            _backgroundWorker.RunWorkerAsync();
        }
        void Timer_Tick(object sender, EventArgs e)
        {
            timeLeft--;
            if (timeLeft <= 0)
            {
                _timer.Stop();
                this.Hide();
                Contact _contact = new Contact();
                _contact.ShowDialog();
                this.Close();
            }
        }
        void BackgroundWorker_ProgressChanged(object sender, ProgressChangedEventArgs e)
        {
            progressBar1.Value = e.ProgressPercentage;
            if (e.ProgressPercentage <= 300)
            {
                _timer.Start();
                this.label3.Text = "Completed ( " + timeLeft + " ) ";
                this.label4.Text = string.Empty;
            }
        }
        void BackgroundWorker_DoWork(object sender, DoWorkEventArgs e)
        {
            for (int i = 0; i <= 300; i++)
            {
                _backgroundWorker.ReportProgress(i);
                System.Threading.Thread.Sleep(200);
            }
        }

"联系人"下面的表格在"欢迎屏幕"之后调用：

public partial class Contact : Form
    {
        const int CP_NOCLOSE_BUTTON = 0x200;
        public Contact()
        {
            InitializeComponent();
        }
        void Contact_Load(object sender, EventArgs e)
        {
            SystemManager.SoundEffect();
        }
        void button1_Click(object sender, EventArgs e)
        {
            this.Hide();
            Loading _loading = new Loading();
            _loading.ShowDialog();
            this.Close();
        }
        protected override CreateParams CreateParams
        {
            get
            {
                CreateParams myCp = base.CreateParams;
                myCp.ClassStyle = myCp.ClassStyle | CP_NOCLOSE_BUTTON;
                return myCp;
            }
        }

我很感激你的回答！

感谢

if (timeLeft <= 0)