Double the payment, triple the total inconvenience

Went to Lanseria to book my nieces flight on 23 September 2018 for 24 September 2018, Booking Reference DZHZXE. Lady who assisted me made an error and duplicated the ticket which lead to me paying for twice the amount. Although I didn't fault her error that she made, I dont understand why I need to wait up till 21 days for such an inconvenience.