ผมอยากทราบว่า การป้องกันไม่ให้ฟอร์มเปิดซ้ำ ๆ กันใน MDI Form C#

private void tsPO_Click(object sender, EventArgs e)
{
    CloseAllChildForm();

    frmPO f = new frmPO();
    f.MdiParent = this;
    f.Show(); 
}

Public Class WForm
    Public Shared Sub ShowForm(Of FormType As {Windows.Forms.Form, New})(ByVal frmMDI As Form)
            For Each ChildForm As Form In frmMDI.MdiChildren
                If TypeOf ChildForm Is FormType Then
                    ChildForm.Focus()
                    ChildForm.WindowState = FormWindowState.Normal
                    Exit Sub
                End If
            Next
            Dim frm As New FormType
            frm.MdiParent = frmMDI
            frm.Show()
    End Sub
End Class

WForm.ShowForm(Of Form1)(mdiForm)