Bug 6292 : Followup 2. several letters where generated if a borrower had overdues...

Bug 6292 : Followup 2. several letters where generated if a borrower had overdues with different due_date triggering the same level

This patch fixes the SQL request giving the list of borrowers

Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>
(cherry picked from commit 1be4678a57200ba895b73e3b3d38f331a9b8cbb4)

Signed-off-by: Chris Nighswonger <chris.nighswonger@gmail.com>
(cherry picked from commit a7ef90ff87ae06e4e1555194b22de155d96fe955)

Signed-off-by: Chris Nighswonger <chris.nighswonger@gmail.com>