I had a functioning 2.16 version of magento, but after I tried to upgrade to the 2.19, all I get is this error message and do not know how to get past it.
Module 'Magento_User' from '/home/www/mysite/app/code/Magento/User' has been already defined in '/home/www/mysite/vendor/magento/module-user'.
Can you confirm if you have the module Magento_Customer on both places? I mean, there's a module Magento_Customer on /home/www/mysite/app/code/ and there is a magento-customer module on /home/www/mysite/vendor/ ?
If the answer is "Yes" can you sahre how you made the upgrade to 2.1.9?
@Damian Culotta Yes, I see the Customer module in both places.
Initially I just ftp'd the files over to the server and tried to run the upgrade from there. Then I used composer install and seemed to pull in all the files, but when I go to the URL it still gives me that error message. I would really just do a complete fresh install, but we have had custom theme work done on the site and did not want to take the chance of losing it. Made a backup of db and files, but would be nice to figure out what is going on with this upgrade.
Basically you have the same module (maybe modules) on both places so Magento is trying to load a module that it was already loaded.
My suggestion is to remove the Magento vendor directory from /app/code (you don't need to delete those files right know, you can just rename that Magento directory).
You can't have the same module on both places.