插入到 ..重复密钥更新无法正常工作

我正在尝试运行此查询：

protected function store_credentials($config_file_path = 'envato_credentials_config.json') {
    $credentials_config = $this->get_envato_config($config_file_path);
    $sql = "INSERT INTO credentials (api_key, username, last_update) VALUES (?, ?, NOW()) ON DUPLICATE KEY UPDATE api_key = values(api_key), username = values(username), last_update = values(last_update)";
    if ($stmt = $this->connect->prepare($sql)) {
        $stmt->bind_param('ss', $credentials_config['API'], $credentials_config['User']);
        $stmt->execute();
        $stmt->close();
    } else {
        return false;
    }
}

但是发生的情况是，我总是在执行时收到重复的条目，如果再次执行，而不是更新它再次运行，添加另外两个条目。为什么会这样？