Bug 27807: (QA follow-up) Unique values only

Bug 27807: (QA follow-up) Unique values only

This patch filters the final _order_by clause such that we only have
unique data fields present and we take the first occurence of a field.

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
(cherry picked from commit 8424f402bed2e71d9080d3adabf5e3255bc79629)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>