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

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>