When XInclude entry contains fallback, xml:id in included fallback file is not correctly added during xmlHashAddEntry, hence "ID %s already defined" is reported and validation fails.

xi-test.xml:
<?xml version="1.0" encoding="utf-8"?>
<article xmlns:xi="http://www.w3.org/2001/XInclude">
  <xi:include href="non-existent.xml" >
    <xi:fallback xmlns:xi="http://www.w3.org/2001/XInclude">
      <xi:include href="existent.xml" />
    </xi:fallback>
  </xi:include>
</article>

existent.xml:
<?xml version="1.0" encoding="utf-8"?>
<info xml:id="info">
  <title>Test</title>
</info>

non-existent.xml doesn't exist. (If it exists, no errors reported).

$ xmllint --xinclude --noout test-xi.xml
warning: failed to load external entity "non-existent.xml"
test-xi.xml:2: element info: validity error : ID info already defined

This is libxml2 2.9.2+dfsg1-3 as found in Ubuntu 15.04. The bug breaks the work of Publican which rely on Perl library XML::LibXML which is the wrapper to libxml2.