Most of the time, these double charges occur because of a mismatch in your billing address and the mailing address associated with your credit card. If you received an error message which resulted in re-submitting your billing info and the attempt failed, the transaction will be listed as pending.
Give your financial institution a call and have them take a closer look at the charges. If the charges are indeed listed as pending, they should drop off in 2-4 business days.
Looking for a refund? You'll need to contact the event organizer.