because I made the reservation online then validated my card in one of their offices. The guy in the office charged my card again. When I went to see him after he told me this can take months !!!! How can I get my money back - it is 100% their fault.

If the charge has not already gone through on the creit card..call the credit card company and put a stop on it. American Express is great with this. Don't know abour others. Have a fit with the airline, keep calling.

Call your credit card company & tell them what happened & let them know that you would like to dispute charges. If you signed for the one with the fellow at the office where you had it validated, then I would suggest that you dispute the charge that was made online.
KLM will have thirty days to validate their charge & if they cannot do so, your credit card company will reclaim those charges for you.
Obviously you signed the charge which was validated at their office, so I would not dispute that charge...you signed off on that one.
I'd do this pronto & continue to pester KLM on the side.

Sqeaky wheel will get the oil!

yes they should be able to refund you your money back and you will need to see the manager call your credit company and report it to them always take actions on what is right go to the higher management

Talk to KLM customer service and ask to speak to the business office. If they can't or won't help you, file a dispute with your credit card company. They will most likely give you all the money in question back to you immediately, then it should only take about 30 days, if not less for the credit card people to resolve completely.

Good Luck.