Thanks for the replies Rod and 'mc'

The refunds do correspond to a refund made by the store operator through the ZC admin. What I'm seeing with the repeated records in the debug logs are IPNs from PayPal telling the cart to record the refund. And it will likely send the IPN for a limited number of times, same as the IPN for payment fail/success is done.

The refund is not recorded in the order in Zen cart, although it does happen at PayPal. I think the error message about the incorrect integer value is probably correct and don't think it's a hack, but then I'm the fix should have worked as well.

I've searched the files for all instances of "TABLE_ORDERS_STATUS_HISTORY" and then looked for any instances associated with inserts into the table. This only exists in the orders.php which is the one I've already edited and then in the authorize.net and linkpoint gateways, which are not enabled and so I haven't edited them.

I don't really know PHP, but I'm going to see if putting '' around the -1 value in swguy's fix will make any difference. I noticed that the same line of code very nearby in orders.php has quotes around the value.

Code:
		  if ($customer_notified === '') { 
		  $customer_notified = '-1';
		  }
I'll see what happens.
Regards
ROb