[schemaserial] be robust against duplicated CWUniqueTogetherConstrain entities arising from bug in the pre 3.18 era
Closes #4619278.
#! /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