multiple "IDs" for constraint linkend

I get these warnings:

$ xsltproc --timing --xinclude --stringparam target.database.document
"olink-chunk.xml" --stringparam current.docid "impnotes" -o test/
docbook-xsl/xhtml/chunk.xsl impnotes.xml
XInclude processing impnotes.xml took 629 ms
Warning: multiple "IDs" for constraint linkend: fdl.
Warning: multiple "IDs" for constraint linkend: gpl.

I have this in impnotes.xml:

<xi:include href="http://www.gnu.org/licenses/fdl-1.3.dbk"
            xmlns:xi="http://www.w3.org/2001/XInclude">
 <xi:fallback><appendix id="fdl"><title>GNU Free Documentation License</title>
   <para>internet connection to the &fsf; is broken</para></appendix>
</xi:fallback></xi:include>
<xi:include href="http://www.gnu.org/licenses/gpl-2.0.dbk"
            xmlns:xi="http://www.w3.org/2001/XInclude">
 <xi:fallback><appendix id="gpl"><title>GNU General Public License</title>
   <para>internet connection to the &fsf; is broken</para></appendix>
</xi:fallback></xi:include>

these <xi:fallback> elements are the only places with id="fdl" and "gpl".
of course, http://www.gnu.org/licenses/gpl-2.0.dbk has the id="gpl" too
and http://www.gnu.org/licenses/fdl-1.3.dbk has an id="fdl".

why am I getting the warnings?
what am I doing wrong?
thanks.

-- 
Sam Steingold <http://sds.podval.org>