When using StAX to apply an XSL template to an XML document, you end up with an XML where default namespace declarations are duplicated.

For instance:

apply test.xsl:

---------------------
<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0">
                <xsl:template match="/">
                               <root xmlns="ns1">
                                               <xsl:call-template name="transform"/>
                               </root>
                </xsl:template>
                <xsl:template name="transform">
                <test xmlns="ns2"><b xmlns="ns2"><c xmlns=""></c></b></test>
                <test xmlns="ns1"><b xmlns="ns2"><c xmlns=""></c></b></test>
                <test><b><c xmlns=""></c></b></test>
                <test xmlns=""><b><c xmlns=""></c></b></test>
                <test xmlns="ns1"><b><c xmlns=""></c></b></test>
                <test xmlns=""/>
                </xsl:template>
</xsl:stylesheet>
-----------------------

to test.xml:

------------------------
<?xml version="1.0" encoding="UTF-8"?><aaa>
    <bbb></bbb>
</aaa>
------------------------

gives you back:

------------------------
<?xml version="1.0" encoding="UTF-8"?>
<root xmlns="ns1" xmlns="ns1">
      <test xmlns="ns2" xmlns="ns2"><b><c></c></b></test>
      <test><b xmlns="ns2" xmlns="ns2"><c></c></b></test>
      <test><b><c></c></b></test>
      <test><b><c></c></b></test>
      <test><b><c></c></b></test>
      <test></test>
</root>
-----------------------------

if you use StAXSource and StAXResult:

    public static void main(String[] args) throws XMLStreamException,
            TransformerConfigurationException, TransformerException {
        TransformerFactory trans = TransformerFactory.newInstance();
        XMLInputFactory input = XMLInputFactory.newFactory();
        XMLOutputFactory output = XMLOutputFactory.newFactory();
        XMLEventReader reader = input.createXMLEventReader(XSLTTest.class.getResourceAsStream("test.xsl"));
        StAXSource source = new StAXSource(reader);
        Templates temp = trans.newTemplates(source);
        StAXResult result = new StAXResult(output.createXMLEventWriter(System.out));
        temp.newTransformer().transform(
                new StAXSource(input.createXMLEventReader(XSLTTest.class.getResourceAsStream("test.xml"))),
                result);
    }

The problem doesn't show up if you use SAX and StreamResult.