[i18ncube] fix crash due to duplicated messages in generated schema.pot
Add a test case and fix regression introduced in e713c47a993d.
Relates to #2811282
Document: cubicweb-docTitle: CubicWeb documentationAuthor: LogilabAbstract: Some base documentation for CubicWeb users and developpersSection: Apps/ProgrammingFormat: HTMLIndex: /usr/share/doc/cubicweb-documentation/index.htmlFiles: /usr/share/doc/cubicweb-documentation/*.html