Running a command inside the wp-cli project directory can require a project config file twice

If you place a YML config file inside the project directory and also it load via a global config file with `WP_CLI_CONFIG_PATH` — and then execute a command inside the directory that contains the YML file — a PHP notice occurs.

This is because the config file is loaded for both global in `$global_config_path` and public in `$project_config_path` when `get_project_config_path()` looks "upward" for more config files in the parent install.

I don't know the best solution, one could be to `array_unique` the required PHP file paths before loading them in Runner. And / or `get_project_config_path()` could inspect the file paths to see if they match the global config path exactly.

Context: in WordPress.com we have a `wp` alias which defines a config file:
`export WP_CLI_CONFIG_PATH='/the-wp-cli-dir/wp-cli.yml`

If I execute a wp-cli command inside `/the-wp-cli-dir/` (or its subdirectories) I see the PHP files required by the YML config load twice.

The YML config file uses this syntax:
`require:/the-wp-cli-dir/some-file.php`