If you run into this error when trying to create a user in mysql, chances are you already have this user account created.

create user 'someuser'@'localhost' identified by 'somepassword';
ERROR 1396 (HY000): Operation CREATE USER failed for 'someuser'@'localhost'

To see all users in your mysql database type:

select user from mysql.user;

+-----------+
| user      |
+-----------+
| mysql.sys |
| root      |
+-----------+

You may can add privileges to a user with the following

grant all privileges on *.* to 'someuser'@'localhost' with grant option;

Or you can drop the user completely with the following

drop user someuser;
flush privileges;

You also may want to restart mysql if the changes do not take place (from the command line not mysql prompt)

mysql.server stop
mysql.server start