The booking system does not allow double bookings, but it is possible for more than one person to have a 'confirmed booking' email for the same seat and time. This is when someone has cancelled a slot, perhaps in error, so that someone else can book it instead.
If you don't remember cancelling your booking, it might be that you, or the person sitting in your seat, pressed the 'cancel booking' link from your confirmation email in error. Please contact us on 0117 928 8000 or library-enquiries@bristol.ac.uk if you would like us to look into it for you.