Overbooked a room by booking.com


I am new on this site and ran already into difficulties.

When I first listed my property is was listed in error as an apartment. Two days ago a lady from booking.com contacted me and I pointed this out to her. I have a B&B with rooms en suit and charge per guest. This morning I get a booking for 2 adults with 3 children sharing for two nights, booking.com has charged them a single rate for 2 night. Besides the point that there is not way that 5 people can stay in one room, the rate is wrong ! I have added the rates as single and double but it seems like the system has only captured the single rate.

What shall I do????

Please help !