Browser Back Button at Checkout over riding orders 2.1.4


We’re having an issue at the checkout stage, customers have pressed “back” button after paying or after going to paypal. The new order replaces the one they have just placed. We have know knowledge or the order in the system,

“Allow a customer to pay the order again in case the transaction was declined” is deselected

We’re using 2.1.4, surely a new submission of on order should create a new order number and not over ride?