Caravan 208B
Air Traffic Control Fully Certified
Situational Awareness
ATC Issue All Types; Conflict Ground Conflict; Critical
Air Traffic Control Issued New Clearance
Human Factors
C172 holding in position on Runway 4L.  C208 cleared to land on Runway 4L. ASDE-X ALARM advised; 'Runway 4L occupied; Go Around'.  C208 advised to; 'change their runway to Runway 4R; cleared to land'. There was a previous go around that had occurred a few minutes prior. The Controller may have not used a pad to  write down the incoming aircraft as a reminder. Controller could improve his/her scan.
Tower Supervisor witnessed an ASDE-X alarm when traffic was cleared to land on an occupied runway forcing a landing runway change; the reporter indicated the Local Controller should have scanned the runway before issuing a clearance.