Happened to me many times with hotels, car rental etc. and double charges were always resulting in the game the banks play (factually it is Visa, MC or AmEx and not the banks that handle the exchange - banks do not touch it at all - but some lesser-honest banks do charge the exchange fee for doing nothing -- when they testified about it in congress the excuse was that the refunds and toll free numbers from a foreign country cost them too [this is hogwash of course but the wise men in congress bought it].
Always banks promptly resolved and refunded whatever it took to make the account. Sometimes it required speaking to a supervisor.
Always disputed the amount of credit with bank based on Fair Credit Billing Act - which in general, dictates that the refund for services must equal the charge for service on the credit card owner's account (unless only part of the service is refunded) and there is absence of exception for fluctuating exchange rates. The theory behind - you must be made whole. But this works in the USA grace to FCBA. Credit card rules look very much different abroad, and in some countries credit cards do what they please.
Of course you need to get to talk to someone at the bank who has basic knowledge of applicable regulations and if not - put it in writing.
In the USA one could also go to Small Claims Court and Hertz would not be able to get off the hook, as it is their actions that cause you damage; FCBA can be overwhelming for small claims court.