public static bool IsRespoOpened = false;
private void btnRespo_ItemClick(object sender, ItemClickEventArgs e) {
  RespoCenterForm r = new RespoCenterForm();
  r.MdiParent = this;

  if (IsRequesterOpened == false) {
    IsRequesterOpened = true;
    r.Show();
  } else {
    Form opened = Application.OpenForms["RespoCenterForm"];
    if (opened == null)
      r.Show();
    else
      opened.Activate();
  }
}