You can't show the same dialog after it is closed. But you should be able to show a different one.
var dlg1 = new Window('dialog','Dialog One');
dlg1.done = dlg1.add('button',undefined,'Done',{name:'cancel'});
var dlg2 = new Window('dialog','Dialog Two');
dlg2.done = dlg2.add('button',undefined,'Done Again',{name:'cancel'});
dlg1.show();
dlg2.show();