Insiemi duplicati di dati inseriti nella tabella

Ho testato il seguente campione e non si sono verificati duplicati:

class Franchises
{
    protected static function getDB()
    {
        $conn = NULL;

        try
        {
            $conn = new PDO("mysql:host=127.0.0.1;dbname=franchises_db", "dbuser", "dbpassword");
            $conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
        }
        catch(PDOException $e)
        {
            echo $e->getMessage();
            exit();
        }
        return $conn;
    } 
    public static function setLeadData($franchises)
    {
       try
       {
            $db = static::getDB();

            $sql = "INSERT INTO leads_franchises SET
                   franchise_id    = :franchise_id,
                   franchisor_id   = :franchisor_id,
                   franchise_name  = :franchise_name";
            $stmt = $db->prepare($sql);

            foreach($franchises as $franchise)
            {
                $result = $stmt->execute([
                    ':franchise_id'   => $franchise['franchise_id'],
                    ':franchisor_id'  => $franchise['franchisor_id'],
                    ':franchise_name' => $franchise['franchise_name']
                ]);
            }

            return $result;
        }
        catch(PDOException $e)
        {
            echo $e->getMessage();
            exit();
        }
    }
}

$vals = [
    [
        "franchise_id" => 3,
        "franchisor_id" => 3,
        "franchise_name" => "Fitness Freaks"
    ],
    [
        "franchise_id" => 4,
        "franchisor_id" => 3,
        "franchise_name" => "Gyms Galore"
    ]
];

Franchises::setLeadData($vals);

e il codice di creazione della tabella lead_franchises è:

CREATE TABLE `leads_franchises` (
    `id` INT(11) NOT NULL AUTO_INCREMENT,
    `franchise_id` INT(11) NULL DEFAULT '0',
    `franchisor_id` INT(11) NULL DEFAULT '0',
    `franchise_name` VARCHAR(128) NULL DEFAULT '',
    PRIMARY KEY (`id`)
)
COLLATE='utf8_general_ci'
ENGINE=InnoDB;