changes2html.pl does not detect duplicate sections in the changes.txt

When reviewing the release artifacts of Lucene 4.3, I noticed that CHANGES.txt contains a section "api changes" 2 times. The changes2html converter should maybe complain about that and fail the build. Otherwise the generated HTML contains the same anchor element two times for one release and the open/close logic breaks (it only open/closes the first one, although you click on the second one).