@Patricia51 I just looked at your listing and you don't have Instant Book turned on so I can't see how someone has booked without your accepting the reservation. Are you sure it wasn't an inquiry instead of a booking request? If the former, all you need to do is answer their questions. If they are asking for more people, then message them back with your answer. There is no commitment by either party.
Should you decide that these would be good guests, you can send them a Pre-approval which then can accept or ignore.
Airbnb encourages guests to contact multiple hosts to check on availabilty, price, special circumstances, etc.
Look at your calendar to see if the dates are blocked off. This will tell you whether it was a confirmed reservation or not.