Hi Andrew
Before confirming the booking go back to the guest and say you want to confirm whether one or two guests are coming as he has only booked for one.
If two guests are coming ask him to update his booking request.
(I normally also ask for the names of all guests on the booking)
Also make sure it is clear in your house rules and description that only the number of guests on the booking will be allowed into the accommodation and that if more guests arrive to stay then have been booked, you may cancel the whole reservation.
Another tip do make sure you agree with a guest before they confirm arrival and departure times so you agree this in advance. (For example I have check in between 6 - 10 p.m. and departure by 11 a.m. This helps ensure guests don't arrive at 'silly O'clock' like 3 a.m.
Helen