BibUpload: duplicate subfields in bibrec_bibxxx

* bibrec_bibxxx tables are not designed to support
  duplicate subfields within the same field of a record.
  However this was not enforced in the table constraints.
  Amends bibuplod to not generate duplicate rows in
  bibrec_bibxxx tables and corrects these table constraints.
  (closes #1714)

Signed-off-by: Samuele Kaplun <samuele.kaplun@cern.ch>
The beauty of the current approach is that most of the time tables are not going to need to be fixed, and hence the `fix_table` needs to basically be never called. If I change approach I will need to basically amend the `fix_table` function. What would be the added value of this?