If you modified the composer.json that comes with Drupal 8 and then moved it above the docroot, you may see the following error when attempting to run drush commands:
Error: Cannot declare class ComposerAutoloaderInitDrupal8, because the name is already in use in /mnt/www/html/<site_name>/docroot/vendor/composer/autoload_real.php, line 5
This is a result of the following line that is added to docroot/vendor/autoload.php as well as the vendor/autoload.php that exists above the docroot.
You'll need to remove the following from your composer.json file:
You should then delete the composer.lock and vendor directory that exist above the docroot and rebuild them with:
composer update --lock