Earlier quoted context omitted.
Heh. We had tons of double billing from payment processors, we never could get a straight answer from the gateway as to why it happened. Most of them came from.a gateway called CyberSource. Also, lots of customers have weird banks where they see both an authorization (I.e. hold) and the actual capture, and they mistake it for two captures. Usually it helps to educate them about what is an authorization.
Man, a bank that doesn't know the difference is a bank I'd be withdrawing my funds from!
Most banks don't even bother showing the customer authorizations of any kind.