if a prefix:NamespaceURI mapping already exists, an attempt to re-declare the prefix:Namespace mapping with the same value throws an Exception.

a unit test is available @:

https://jaxp-sources.dev.java.net/source/browse/jaxp-sources/jaxp-ri/src/unit-test/bug6467808/Bug.java

the fix is avialble @:

https://jaxp-sources.dev.java.net/source/browse/jaxp-sources/src/com/sun/org/apache/xalan/internal/xsltc/trax/Attic/SAX2DOM.java?only_with_tag=jaxp-1_3-j2se-5_0-branch