It's similar to the double opt out that one has to do in Portugal sometimes. This would go against the spirit of the DCC rules that state the customer shouldn't be corralled toward a particular decision, which an, "Are you sure? Are you really sure?" implementation seems to do. Furthermore, the rate has to be disclosed to comply with the Visa/MC implementation rules around DCC.

Of course, reading this thread and its predecessors will give many examples of DCC that aren't in compliance with the payment network rules.