We have an eCommerce website that takes credit card payments, everything is fine, however sometimes a user enters their details, gets approved and moves on to the Complete page - they then hit back and hit Submit AGAIN (they get a duplicate transaction error of course), but my question is - What is the best way to discourage this behavior?
My initial thoughts are
Grey the button out and add auxiliary text or change the button text - to "Processing..."
Add an "Intermediate" page that shows the text "Processing" with an ajax spinner and some additional text discouraging hitting back or closing
Is there a best practice for this schenario?