Bug 20462: Ensure batch item deletion will not delete the same item twice

Bug 20462: Ensure batch item deletion will not delete the same item twice

This has been done on bug 9509 when barcodes are scaned, but not when using a text file.

Test plan (stolen from Barton on the bug report):
1) Create a text file containing duplicate barcodes

Here's a quick way to do that, substitute INSTANCE for the instance name of your test server:

sudo koha-mysql INSTANCE <<< "select barcode from items where barcode is not null limit 3" | sed '1d;p' > /tmp/dup_barcodes.txt

2) Go to Home › Tools › Batch item deletion
3) Click 'Choose File', select the file you created in step 1.
4) Click 'Continue'
5) Note that duplicate items appear in the list of items to batch delete
6) Check 'Delete records if no items remain'
7) This will trigger a software error

Note that entering the barcodes via the 'scan items one by one' text box removes duplicates in the list, so this only causes problems when uploading a file.

Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
(cherry picked from commit 1f4baf31bb19c615f43771efa3fac7e2333d3622)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>