No the guest is choosing to book for one guest @David1160 . If this is happening to you a lot, I would make sure your house rules clearly state only those who book can stay. And message guests on booking to remind them on this house rule.
When people arrive with two, you can either ask the second person to leave or contact Airbnb and ask them to be added to the booking and send a request for the additional rate for the second person for the guest to pay immediately.
If they don't pay ask Airbnb to cancel the booking.