LSSerialiser.write() produces an incorrect output.

Details:
While serializing following XML Document:
-------------------------------
<?xml version=\"1.0\"?>
<ROOT>
  <ELEMENT1>
  </ELEMENT1>
  <ELEMENT2>
  </ELEMENT2>
</ROOT>
-------------------------------
we get the following output:
-------------------------------
<?xml version="1.0" encoding="UTF-16"?>
<?xml version="1.0" encoding="UTF-16"?>
<ROOT>
  <ELEMENT1/>
  <ELEMENT2/>
</ROOT>
-------------------------------
Document itself is OK, but <?xml...?> header is
printed twice.

Run following example to simply reproduce this failure:
---------8<----------Test.java----------------------
import org.w3c.dom.ls.*;
import java.io.*;
import org.xml.sax.*;
import org.w3c.dom.*;
import javax.xml.parsers.*;

class Test {
  public static void main(String args[]) {
    String xml = "<?xml version=\"1.0\"?><ROOT><ELEMENT1></ELEMENT1><ELEMENT2></ELEMENT2></ROOT>";
    DocumentBuilder builder = null;
    Document doc = null;
    try {
      DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
      builder = factory.newDocumentBuilder();
    } catch (ParserConfigurationException e) {
      e.printStackTrace();
    }
    try {
      doc = builder.parse(new StringBufferInputStream(xml));
    } catch (SAXException e) {
      e.printStackTrace();
    } catch (IOException e) {
      e.printStackTrace();
    }
    DOMImplementation impl = doc.getImplementation();
    DOMImplementationLS implLS = (DOMImplementationLS) impl.getFeature("LS","3.0");
    LSInput src = implLS.createLSInput();
    src.setStringData(xml);
    LSParser parser = implLS.createLSParser(DOMImplementationLS.MODE_SYNCHRONOUS,"http://www.w3.org/2001/XMLSchema");
    LSSerializer writer = implLS.createLSSerializer();
    Document xmldoc = parser.parse(src);
    String result = writer.writeToString(xmldoc);
    System.out.println(result);
  }
}
---------8<----------Test.java----------------------


We have a number of JCK test failures due to this bug.
For example: api/org_w3c/dom/ls/LSSerializer/index.html#write[write0001]