Great discussion guys! Very educational and informative.
I don't know if this is much use to you, but I find it useful to use Javascript to disable a submit/button after clicking. It's hard to double-click a button that's grayed out. Of course, the user may have Javascript disabled, but in this day and age, it's quite unlikely and anyway, they will probably have other problems too. 😊 Of course, it all depends on your accessibility requirements too.
Something like this should do it:
<input type="submit" name="pay" value=" Pay " onclick="this.disabled=true;document.myform.submit() >
