Redis configuration for Craft CMS

app.php

<?php
/**
 * Yii Application Config
 *
 * Edit this file at your own risk!
 *
 * The array returned by this file will get merged with
 * vendor/craftcms/cms/src/config/app/main.php and [web|console].php, when
 * Craft's bootstrap script is defining the configuration for the entire
 * application.
 *
 * You can define custom modules and system components, and even override the
 * built-in system components.
 */

return [
    'components' => [
        'cache'         => [
            'class'     => 'yii\redis\Cache',
            'defaultDuration' => 86400,
            'keyPrefix' => getenv('REDIS_KEYPREFIX').'_cache_',
            'redis'     => [
                'hostname' => getenv('REDIS_HOST'),
                'port'     => getenv('REDIS_PORT'),
                'database' => 0, 
                'password' => getenv('REDIS_PASSWORD') // Optional
            ]
        ],
        'session' => [
            'class' => 'yii\redis\Session',
            'as sessionBehavior' => [
                'class' => 'craft\behaviors\SessionBehavior'
            ],
            'keyPrefix' => getenv('REDIS_KEYPREFIX').'_session_',
            'redis' => [
                'hostname' => getenv('REDIS_HOST'),
                'port'     => getenv('REDIS_PORT'),
                'database' => 1,
                'password' => getenv('REDIS_PASSWORD') // Optional
            ]
        ],
    ],
    'modules' => [
        'siteModule' => \modules\Module::class,
    ],
    'bootstrap' => [
        'siteModule',
    ],
];

Is it necessary to define the redis config twice in the cache and session component?

Can't you define redis as a component itself, given the class for both cache and session are both point to Redis?