BibUpload: fix for inserting duplicate subfields
BibUpload: fix for inserting duplicate subfields

* Avoids an integrity error when a field contains a duplicate
  subfield with the same value.

Signed-off-by: Javier Martin Montull <ea6b80e26c65acc242d8916fb6eefa01b123cb69@cern.ch>
Reviewed-by: Jiri Kuncar <2bdffcb416fed3002465740d19a053a39918cbe5@cern.ch>

inveniosoftware/invenio,inveniosoftware/invenio,tiborsimko/invenio,tiborsimko/invenio