hi @Íris2,
never ever cancel a booking from your side!
It's hard to imagine that you really got a double booking for one listing only, but if you did, you should contact airbnb in such a case and have staff cancel one booking for you. In that case, there is no cost nor penalty.
You should still contact them, explain what happened, let them find yout, where the error occured and get the penalty removed. If it was a technical error, you will not be punished for it, but you have to tell a real person at airbnb about it. If you only cancel, it's handled by computer programs and nobody knows about it. You got an automatic fine.
This guide may be helpful: https://community.withairbnb.com/t5/Community-Help/Contact-Airbnb-A-Community-Help-Guide/m-p/16165#M...