Magento 2.1.9 to Magento 2.2.1 upgrade error

Question

I am trying to update to latest version but when I run upgrade command from the console than its showing below upgrade error.

Answer 1

Please follow these steps, and then try

rm -rf app/code/Magento/ var/* vendor/*
chmod 777 -R *
composer update && composer install
php -f bin/magento setup:static-content:deploy
find . -type d -exec chmod 775 {}
find . -type f -exec chmod 660 {}
chmod u+x bin/magento
bin/magento maintenance:disable
bin/magento cache:clean

Refer this for more info : Module 'Magento_CatalogInventory' has already been defined

Answer 2

I upgraded from Magento 2.1.9 to Magento 2.2.1 using these commands

composer require magento/product-community-edition 2.2.1 --no-update
composer update
rm -rf var/di var/generation
php bin/magento cache:flush
php bin/magento setup:upgrade
php bin/magento setup:di:compile
php bin/magento indexer:reindex

After upgrade, check your Magento version with the following command:

php bin/magento --version

Reference