After investigated staff who assigned check in duty on flight SL8511 29 JAN 16 found that, my staff confirmed that passenger contact for asking about the flight after check-in counter closed around 08:00 AM during we open check-in counter of flight SL8515. Firstly, our staff cannot find passenger in flight then use surname retrieve existing reservation found in SL8511/29 JAN 16 which departed. Passenger inform our team they long spending time for waiting at Bangkok air counter caused they don’t know. Staff inform passenger to make a refund by CF90 PCT as normal. Moreover, on the system the last passenger on that flight is the passenger who seat on number 19E which has been checked at 06:30 am sequent number 193 by CNXGPP.