Bug 30003: Prevent double up of point-of-sale items

This patch prevents the double submission of point-of-sale items when form validation fails at the first submission

Test plan
 1.  Navigate to /cgi-bin/koha/pos/pay.pl
 2.  Add an item from 'items to purchase'
 3.  Under 'collect payment', click submit without altering the form
 4.  See 'This field is required.'
 5.  Now, fill a value in 'Amount tendered' that satisfies or exceeds 'Amount being paid'
 6.  The payment should succeed
 7.  Navigate to /cgi-bin/koha/pos/register.pl
 8.  See duplicate lines in Transactions
 9.  Apply patch
10.  Repeat 2-6
11.  Navigate to /cgi-bin/koha/pos/register.pl
12.  Confirm only a single line was added

Signed-off-by: Sally <sally.healey@cheshiresharedservices.gov.uk>

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>

koha-tmpl/intranet-tmpl/prog/en/modules/pos/pay.tt

@@ -402,6 +402,14 @@
         });
 
         $('#payForm').submit(function(e){
+            // first, clear stale sales 'rows' from the payForm
+            if($('input[name="sales"]').length > 0) {
+                $('input[name="sales"]').each(function() {
+                    $(this).remove();
+                });
+            }
+
+            // now, process the current & fresh contents of the sale_table
             if (change != undefined && change.innerHTML > 0.00 && !checked) {
                 e.preventDefault();
                 $("#confirm_change_form").modal("show");