Bug 5144: Duplicate holds allowed if patron clicks back button after placing hold

Bug 5144: Duplicate holds allowed if patron clicks back button after placing hold

Koha is currently not engineered to handle multiple holds per record.
Until such time that is does, we should not allow them to be created.

Test Plan:
1) Apply this patch
2) Log in to the opac
3) Place a hold
4) Hit the back button on your browser
5) Place the hold again
6) Note the new message

Signed-off-by: David Kuhn <kuhn@monterey.org>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>