xml Xpath

[mickey0]

Hello,
I have a xml-like file as this:

<xml...............>
  <mytag id="one"............/>
  <mytag id="two"............/>
  <mytag id="one"............/> //--error
  <mytag id="three"............>
            <mytag id="three.one"............/>
            <mytag id="three.one"............/> //--error
  </mytag>
 

My xml can't contains duplicate id value; so to check if there are any I wrote this:

DocumentBuilderFactory docBuilderFactory = DocumentBuilderFactory.newInstance();
DocumentBuilder docBuilder = docBuilderFactory.newDocumentBuilder();
Document doc = docBuilder.parse (new File( fileName ));
XPath xp = XPathFactory.newInstance().newXPath();
NodeList nl = (NodeList) xp.evaluate("/xml/state/@id", doc,  XPathConstants.NODESET);
Set<String> allowedIds = new HashSet<String>();
 for ( int i = 0; i < nl.getLength(); i++ ) {
                  //pus in Set..........
                  //nl is empty!!! Why?
}
 

But nl is empty. Where am I doing the mistake?
thanks,