If a configuration file already exists that contains a MySQL username/password, and user attempts to execute:

pmm-admin add mysql:metrics --create-user --port

We should pause, and throw warning message:

Password already defined for user 'pmm', if you continue you will be generating a new password that may break access to MySQL and require manual intervention. If you'd like to continue, execute with --force

Or something to this effect. This can be duplicated for any environment that is running more than one instance of MySQL - perhaps on different ports (MySQL::Sandbox). Each time you execute with --create-user you define a new password which is stored to the configuration file, so earlier executions are overwritten.

Until this improvement is released, the workaround for users is to check the password:

pmm-admin show-passwords

And ensure that all locally running instances are configured for the same password.