[i18ncube] fix crash due to duplicated messages in generated schema.pot
Add a test case and fix regression introduced in e713c47a993d.
Relates to #2811282
Schema of the data
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First take a look at the data schema_ then try to remember that you are browsing
through a heap of data by applying stylesheets to the results of queries. This
site is not a content management system with items placed in folders. It is an
interface to a database which applies a view to retreived data.
.. _schema: schema