rake routesでwarningが出る

Question

http://clc.gonna.jp/?p=1306

上記のサイトを参考にして、devise token authを使っているのですが、
rake routesコマンドを実行すると、

$ rake routes
/home/rails/devise_token_auth/vendor/bundle/ruby/2.4.0/gems/rake-12.0.0/lib/rake/file_utils.rb:9: warning: already initialized constant FileUtils::RUBY
/usr/local/rbenv/versions/2.4.1/lib/ruby/gems/2.4.0/gems/rake-12.0.0/lib/rake/file_utils.rb:9: warning: previous definition of RUBY was here
/home/rails/devise_token_auth/vendor/bundle/ruby/2.4.0/gems/rake-12.0.0/lib/rake/file_utils.rb:108: warning: already initialized constant FileUtils::LN_SUPPORTED
/usr/local/rbenv/versions/2.4.1/lib/ruby/gems/2.4.0/gems/rake-12.0.0/lib/rake/file_utils.rb:108: warning: previous definition of LN_SUPPORTED was here
/home/rails/devise_token_auth/vendor/bundle/ruby/2.4.0/gems/rake-12.0.0/lib/rake/file_utils_ext.rb:16: warning: already initialized constant Rake::FileUtilsExt::DEFAULT
/usr/local/rbenv/versions/2.4.1/lib/ruby/gems/2.4.0/gems/rake-12.0.0/lib/rake/file_utils_ext.rb:16: warning: previous definition of DEFAULT was here
WARNING: Possible conflict with Rake extension: String#ext already exists
WARNING: Possible conflict with Rake extension: String#pathmap already exists
/home/rails/devise_token_auth/vendor/bundle/ruby/2.4.0/gems/rake-12.0.0/lib/rake/file_list.rb:43: warning: already initialized constant Rake::FileList::ARRAY_METHODS
/usr/local/rbenv/versions/2.4.1/lib/ruby/gems/2.4.0/gems/rake-12.0.0/lib/rake/file_list.rb:43: warning: previous definition of ARRAY_METHODS was here
/home/rails/devise_token_auth/vendor/bundle/ruby/2.4.0/gems/rake-12.0.0/lib/rake/file_list.rb:46: warning: already initialized constant Rake::FileList::MUST_DEFINE
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/usr/local/rbenv/versions/2.4.1/lib/ruby/gems/2.4.0/gems/rake-12.0.0/lib/rake/version.rb:7: warning: previous definition of NUMBERS was here
/home/rails/devise_token_auth/vendor/bundle/ruby/2.4.0/gems/rake-12.0.0/lib/rake/linked_list.rb:109: warning: already initialized constant Rake::LinkedList::EMPTY
/usr/local/rbenv/versions/2.4.1/lib/ruby/gems/2.4.0/gems/rake-12.0.0/lib/rake/linked_list.rb:109: warning: previous definition of EMPTY was here
/home/rails/devise_token_auth/vendor/bundle/ruby/2.4.0/gems/rake-12.0.0/lib/rake/scope.rb:40: warning: already initialized constant Rake::Scope::EMPTY
/usr/local/rbenv/versions/2.4.1/lib/ruby/gems/2.4.0/gems/rake-12.0.0/lib/rake/scope.rb:40: warning: previous definition of EMPTY was here
/home/rails/devise_token_auth/vendor/bundle/ruby/2.4.0/gems/rake-12.0.0/lib/rake/promise.rb:11: warning: already initialized constant Rake::Promise::NOT_SET
/usr/local/rbenv/versions/2.4.1/lib/ruby/gems/2.4.0/gems/rake-12.0.0/lib/rake/promise.rb:11: warning: previous definition of NOT_SET was here
/home/rails/devise_token_auth/vendor/bundle/ruby/2.4.0/gems/rake-12.0.0/lib/rake/application.rb:12: warning: already initialized constant Rake::CommandLineOptionError
/usr/local/rbenv/versions/2.4.1/lib/ruby/gems/2.4.0/gems/rake-12.0.0/lib/rake/application.rb:12: warning: previous definition of CommandLineOptionError was here
/home/rails/devise_token_auth/vendor/bundle/ruby/2.4.0/gems/rake-12.0.0/lib/rake/application.rb:40: warning: already initialized constant Rake::Application::DEFAULT_RAKEFILES
/usr/local/rbenv/versions/2.4.1/lib/ruby/gems/2.4.0/gems/rake-12.0.0/lib/rake/application.rb:40: warning: previous definition of DEFAULT_RAKEFILES was here
/home/rails/devise_token_auth/vendor/bundle/ruby/2.4.0/gems/rake-12.0.0/lib/rake/task_arguments.rb:107: warning: already initialized constant Rake::EMPTY_TASK_ARGS
/usr/local/rbenv/versions/2.4.1/lib/ruby/gems/2.4.0/gems/rake-12.0.0/lib/rake/task_arguments.rb:107: warning: previous definition of EMPTY_TASK_ARGS was here
/home/rails/devise_token_auth/vendor/bundle/ruby/2.4.0/gems/rake-12.0.0/lib/rake/invocation_chain.rb:54: warning: already initialized constant Rake::InvocationChain::EMPTY
/usr/local/rbenv/versions/2.4.1/lib/ruby/gems/2.4.0/gems/rake-12.0.0/lib/rake/invocation_chain.rb:54: warning: previous definition of EMPTY was here
/home/rails/devise_token_auth/vendor/bundle/ruby/2.4.0/gems/rake-12.0.0/lib/rake/early_time.rb:20: warning: already initialized constant Rake::EARLY
/usr/local/rbenv/versions/2.4.1/lib/ruby/gems/2.4.0/gems/rake-12.0.0/lib/rake/early_time.rb:20: warning: previous definition of EARLY was here
/home/rails/devise_token_auth/vendor/bundle/ruby/2.4.0/gems/rake-12.0.0/lib/rake/late_time.rb:16: warning: already initialized constant Rake::LATE
/usr/local/rbenv/versions/2.4.1/lib/ruby/gems/2.4.0/gems/rake-12.0.0/lib/rake/late_time.rb:16: warning: previous definition of LATE was here
/home/rails/devise_token_auth/vendor/bundle/ruby/2.4.0/gems/rake-12.0.0/lib/rake/backtrace.rb:3: warning: already initialized constant Rake::Backtrace::SYS_KEYS
/usr/local/rbenv/versions/2.4.1/lib/ruby/gems/2.4.0/gems/rake-12.0.0/lib/rake/backtrace.rb:3: warning: previous definition of SYS_KEYS was here
/home/rails/devise_token_auth/vendor/bundle/ruby/2.4.0/gems/rake-12.0.0/lib/rake/backtrace.rb:4: warning: already initialized constant Rake::Backtrace::SYS_PATHS
/usr/local/rbenv/versions/2.4.1/lib/ruby/gems/2.4.0/gems/rake-12.0.0/lib/rake/backtrace.rb:4: warning: previous definition of SYS_PATHS was here
/home/rails/devise_token_auth/vendor/bundle/ruby/2.4.0/gems/rake-12.0.0/lib/rake/backtrace.rb:7: warning: already initialized constant Rake::Backtrace::SUPPRESSED_PATHS
/usr/local/rbenv/versions/2.4.1/lib/ruby/gems/2.4.0/gems/rake-12.0.0/lib/rake/backtrace.rb:7: warning: previous definition of SUPPRESSED_PATHS was here
/home/rails/devise_token_auth/vendor/bundle/ruby/2.4.0/gems/rake-12.0.0/lib/rake/backtrace.rb:11: warning: already initialized constant Rake::Backtrace::SUPPRESSED_PATHS_RE
/usr/local/rbenv/versions/2.4.1/lib/ruby/gems/2.4.0/gems/rake-12.0.0/lib/rake/backtrace.rb:11: warning: previous definition of SUPPRESSED_PATHS_RE was here
/home/rails/devise_token_auth/vendor/bundle/ruby/2.4.0/gems/rake-12.0.0/lib/rake/backtrace.rb:15: warning: already initialized constant Rake::Backtrace::SUPPRESS_PATTERN
/usr/local/rbenv/versions/2.4.1/lib/ruby/gems/2.4.0/gems/rake-12.0.0/lib/rake/backtrace.rb:15: warning: previous definition of SUPPRESS_PATTERN was here
/home/rails/devise_token_auth/vendor/bundle/ruby/2.4.0/gems/rake-12.0.0/lib/rake.rb:69: warning: already initialized constant FileList
/usr/local/rbenv/versions/2.4.1/lib/ruby/gems/2.4.0/gems/rake-12.0.0/lib/rake.rb:69: warning: previous definition of FileList was here
/home/rails/devise_token_auth/vendor/bundle/ruby/2.4.0/gems/rake-12.0.0/lib/rake.rb:70: warning: already initialized constant RakeFileUtils
/usr/local/rbenv/versions/2.4.1/lib/ruby/gems/2.4.0/gems/rake-12.0.0/lib/rake.rb:70: warning: previous definition of RakeFileUtils was here
                      Prefix Verb     URI Pattern                            Controller#Action
        new_api_user_session GET      /api/auth/sign_in(.:format)            devise_token_auth/sessions#new
            api_user_session POST     /api/auth/sign_in(.:format)            devise_token_auth/sessions#create
    destroy_api_user_session DELETE   /api/auth/sign_out(.:format)           devise_token_auth/sessions#destroy
       new_api_user_password GET      /api/auth/password/new(.:format)       devise_token_auth/passwords#new
      edit_api_user_password GET      /api/auth/password/edit(.:format)      devise_token_auth/passwords#edit
           api_user_password PATCH    /api/auth/password(.:format)           devise_token_auth/passwords#update
                             PUT      /api/auth/password(.:format)           devise_token_auth/passwords#update
                             POST     /api/auth/password(.:format)           devise_token_auth/passwords#create
cancel_api_user_registration GET      /api/auth/cancel(.:format)             api/auth/registrations#cancel
   new_api_user_registration GET      /api/auth/sign_up(.:format)            api/auth/registrations#new
  edit_api_user_registration GET      /api/auth/edit(.:format)               api/auth/registrations#edit
       api_user_registration PATCH    /api/auth(.:format)                    api/auth/registrations#update
                             PUT      /api/auth(.:format)                    api/auth/registrations#update
                             DELETE   /api/auth(.:format)                    api/auth/registrations#destroy
                             POST     /api/auth(.:format)                    api/auth/registrations#create
     api_auth_validate_token GET      /api/auth/validate_token(.:format)     devise_token_auth/token_validations#validate_token
            api_auth_failure GET      /api/auth/failure(.:format)            devise_token_auth/omniauth_callbacks#omniauth_failure
                             GET      /api/auth/:provider/callback(.:format) devise_token_auth/omniauth_callbacks#omniauth_success
                             GET|POST /omniauth/:provider/callback(.:format) devise_token_auth/omniauth_callbacks#redirect_callbacks
            omniauth_failure GET|POST /omniauth/failure(.:format)            devise_token_auth/omniauth_callbacks#omniauth_failure
                             GET      /api/auth/:provider(.:format)          redirect(301)

このように、warningが出ます。

/home/rails/devise_token_auth/vendor/bundle/ruby/2.4.0/gems/rake-12.0.0/lib/rake/file_utils.rb

のrequire "fileutils" を削除してみたりしたのですが、ダメでした。

どうすれば、消えるでしょうか。

よろしくお願いします。