Hi,
If both the reservations are for the same place, then you should contact the host and tell them there was a payment error which caused you to book it twice. They will either cancel one of the reservations or cancel both and have you rebook it.
If both reservations are different, I would contact the host of the location you prefer to stay in and ask if they received the payment and booking. If so, then cancel the other reservation. If not, rebook your stay and cancel then cancel your second reservation.
Hope this clears things up. Best of luck!