[i18ncube] fix crash due to duplicated messages in generated schema.pot
Add a test case and fix regression introduced in e713c47a993d.
Relates to #2811282
#! /bin/sh -e#if [ "$1" = configure ]; then if which install-docs >/dev/null 2>&1; then install-docs -i /usr/share/doc-base/cubicweb-doc fifi#DEBHELPER#exit 0