如何在C#的MDI窗体中同样的子窗体只显示一次

/// <summary>
/// 打开窗体
/// </summary>
/// <param name="mdiChildTypeString"></param>
/// <returns></returns>     private void ShowWindow(string mdiChildTypeString)
{

     Form currentMdiChild = null;

    if (!IsMDIChildFormExist(mdiChildTypeString))
     {
         Assembly assembly = Assembly.GetExecutingAssembly();
         Type typForm = assembly.GetType(mdiChildTypeString);

        //关于InvokeMember用法，不明白的可以查看：http://msdn.microsoft.com/zh-cn/library/de3dhzwy(VS.80).aspx
         Object mdiChild = typForm.InvokeMember(
            null,
             BindingFlags.DeclaredOnly | BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance | BindingFlags.CreateInstance,
            null,
            null,
            null);

        if (mdiChild != null)
         {
             currentMdiChild = mdiChild as Form;
             currentMdiChild.MdiParent = this;
             currentMdiChild.WindowState = FormWindowState.Maximized;
             currentMdiChild.Show();
             currentMdiChild.Focus();
         }

     }

}

/// <summary>
/// 检查MDI子窗体是否存在
/// </summary>
/// <param name="mdiChildTypeString"></param>
/// <returns></returns>        
private bool IsMDIChildFormExist(string mdiChildTypeString)
{
     Form currentMDIChild = null;
    foreach (Form mdiChildFrm in this.MdiChildren)
     {
        if (mdiChildFrm.GetType().ToString() == mdiChildTypeString)
         {
             currentMDIChild = mdiChildFrm;
            break;
         }
     }

    if (currentMDIChild != null)
     {
        if (currentMDIChild.WindowState == FormWindowState.Minimized)
         {
             currentMDIChild.WindowState = FormWindowState.Normal;
         }
         currentMDIChild.TopMost = true;
         currentMDIChild.Activate();
         currentMDIChild.Focus();
        return true;
     }
    else
     {
        return false;
     }

}

//调用方法如下

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

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