Zephyr doesn't report an error on a malformed XML document like:

  <foo xmlns:xml='http://www.w3.org/XML/1998/namespace&#39; xmlns:xml='http://www.w3.org/XML/1998/namespace&#39; />

(Notice the double declaration of xmlns:xml)

I added a JUnit test case in $ZEPHYR/test/XMLStreamReader/DoubleXmlnsTest for your reference.

I'm filing this bug because it's desirable for the JAXP team to fix this, but it's not blocking us.