$ wp plugin install wordpress-importer --activate

  Activating 'wordpress-importer'...
  Success: Plugin already installed.

  Warning: wordpress-importer: Plugin already installed.
  Warning: Plugin 'wordpress-importer' is already active.

Warning is not correct. The WP installation is in the state we expected to find it (although our requested operation wasn't needed). If double installation/activation intent do not hurt [eg no double-execution of install() function], then it should not be treated as a warning, but simply a notice.