Hi @루나0
I am sorry, but if you made a booking, depending how far away the booking is, you may not be able to get a refund. If the booking is for tomorrow, for example, there is little chance of the guest booking the room to someone else.
How were you "Not aware" of the second booking?
You can try to contact Airbnb on the phone
Here you have telephone numbers
https://community.withairbnb.com/t5/Help/Contact-Airbnb-A-Community-Help-Guide-UPDATED/td-p/413245