创建新的MessageBox窗口前，先关掉之前已经创建好的

procedure TForm2.Timer1Timer(Sender: TObject);

var

  h:HWND;

begin

  h := FindWindow(nil,PAnsiChar('123'));

  if h > 0 then

  begin
    //PostMessage(h,WM_SYSCOMMAND, SC_CLOASE,0);//现在这样写法不管用

    SendMessage(h,WM_SYSCOMMAND, SC_CLOSE,0);

  end;

  if Application.messageBOX(PChar('是否继续？'), PChar('123'), MB_OKCANCEL + MB_ICONINFORMATION + MB_DEFBUTTON2 + MB_TOPMOST) = IDCANCEL then

  begin

    Exit;

  end

end;

来源：https://www.cnblogs.com/OSKnown/p/9628237.html