[nailer]

The back button exists, banks can not refuse to allow its use. All they can do is warn against its use.

Have a unique hidden input for every form. If a user goes back, regenerate the form.

If a form is somehow resubmitted, print a nice message saying saying that transaction was already submitted.

[7Z7]

My bank immediately logs me out if I hit the back button (instead of using their "go back" link) at any time during a logged-in session. That's whether I'm making a payment, looking at my statements, setting up direct-debits etc, or just sitting ambiently on the home-screen. It's incredibly annoying.