$arr = [['name' => 'ford', 'count' => 2],
    ['name' => 'ford', 'count' => 8],
    ['name' => 'lada', 'count' => 9],
    ['name' => 'lada', 'count' => 2]];

$result = [];
foreach($arr as $car) {
    if (!isset($result[$car['name']]) || $result[$car['name']] < $car['count']) {
        $result[$car['name']] = $car['count'];
    }
}

$time = '11:00:00';
$duration = '03:30:00';

$newTime = strtotime($time) + strtotime($duration) - strtotime("00:00:00");
$newTime = date("H:i:s", $newTime);
echo $newTime;