Arrays

// horários que são exibidos para os clientes
$hours        = array('08:00','08:30','09:00','09:30','10:00','10:30','11:00','11:30','12:00','12:30','13:00','13:30','14:00');

// horários que os clientes marcaram
$schedule     = array('08:00','08:00','09:00','09:00','09:00','10:00','10:00','10:00','10:00','11:00','11:00','11:00','11:00','11:00' );

// ao chegar nesse valor de consultas agendada por horário é retirado o horário do array hours
$chooseNumber = 5;

$count        = array_count_values( $schedule );
foreach( $count as $number => $value )
{

    if( $value > 1 )
    {
        $schedules[$value] = $number;

        if( $value == $chooseNumber )
        {
             $newHours[$value] = $number;
        }

     }


}
echo "<pre>";
print_r( $schedules );
echo "</pre>";

echo "<pre>";
print_r( array_diff( $hours, $newHours ) );
echo "</pre>";