It is not possible to cancel a tenant ePay while it is being processed. You must wait for the payment to complete.
Two Possible Outcomes
The duplicate payment clears — Once the payment is deposited into your bank account, refund or credit the tenant for the overpayment in DoorLoop.
The duplicate payment is reversed — Most likely due to insufficient funds. In this case, no payment was collected, but the tenant will be charged a reversed payment fee.
Regardless of the outcome, you cannot cancel a tenant ePay mid-processing.