title: Get duplicate attributes in xml using schematron
body: <p>I am writing few checks for XML file using schematron.</p>
  
  <p>I want to write down a check so that there must be no duplicate attribute <code>'id'</code> in any tag across the xml.<br>
  Please note, attribute <code>'id'</code> can occur in any element across the xml.<br>
  I found something related to this but that was case for siblings only which used preceding-sibling function.</p>
  
  <p>Please suggest.</p>