[xml/sgml-pkgs] Bug#766884: libxml2: lots of "validity error : ID ... already defined" errors with xmllint --valid

[Vincent Lefevre]

As said on https://bugzilla.gnome.org/show_bug.cgi?id=737840#c4 this
affects the building process of KDE documentation.

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Vincent Lefèvre <vincent at vinc17.net> - Web: <https://www.vinc17.net/>
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Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)