Fatal error: Cannot redeclare GuzzleHttp afetr installing it using Composer

Question

I've installed manually the SDK package of Amazon AWS a long time ago in a website. And I'm calling it like this:

require $_SERVER['DOCUMENT_ROOT'].'/aws/aws-autoloader.php';
 use Aws\S3\S3Client;
 use Aws\Exception\AwsException;

I completely forgot about it, but I've started to use composer now, and I install a package that uses GuzzleHttp, because I get this error:

Fatal error: Cannot redeclare GuzzleHttp\describe_type() (previously declared in /home/ana/public_html/vendor/guzzlehttp/guzzle/src/functions.php:16) in /home/ana/public_html/aws/GuzzleHttp/functions.php on line 41

This is how my composer.json file looks now:

{
  "require": {
      "phpmailer/phpmailer": "^6.4",
      "getbrevo/brevo-php": "1.x.x" <------- the new package that caused trouble
  }
}

Brevo app has GuzzleHttp as a dependency... what can I do?