If the guest choose "offline payment", "custom gateway" or credit card collection Depending the payment method the the system may send out two confirmations.
The first confirmation message is sent out as soon as the guest clicks "Confirm Booking" to confirm their request.
A second message containing the payment instruction message will be sent if a payment instruction for this method is entered in SETTINGS->PROPERTIES->PAYMENT COLLECTION->OFFLINE PAYMENT/CREDIT CARD CUSTOM GATEWAY after the guest completed the payment, entered their card details or clicked continueaccepted to pay offline.
If you do not want to send two confirmation messages set Non Payment Booking Status" = Cancelled.