如何在C#的MDI窗体中同样的子窗体只显示一次

 private void toolStripButton1_Click(object sender, EventArgs e)
        {
            ShowWindow(typeof(Form1).ToString());          
           
        }

private void toolStripButton2_Click(object sender, EventArgs e)
        {
            ShowWindow(typeof(Form2).ToString());  
        }