Invalid route name, already in use: 'new_user_session'のエラー解決方法

環境

Mac OS Catalina
Ruby 2.7.1
Rails 6.0.3.2

現象

ログイン機能用にdeviseを導入しrouteを設定。
その後、Rails routesを実行すると下記のエラーが出た。

Invalid route name, already in use: 'new_user_session'  (ArgumentError)
You may have defined two routes with the same name using the `:as` option, or you may be overriding a route already defined by a resource with the same naming. For the latter, you can restrict the routes created with `resources` as explained here: 
https://guides.rubyonrails.org/routing.html#restricting-the-routes-created

どうやら、'new_user_session'という名前のルートが既に使われているぞ！と言ってる様子。

現象発生時のルートは下記の通り。

routes.rb

Rails.application.routes.draw do
  devise_for :users
  # For details on the DSL available within this file, see https://guides.rubyonrails.org/routing.html

  root to: 'toppages#index'

  resources :users, only: [:index, :show, :edit, :update, :destroy] do
    member do
      get :followings
      get :followers
      get :likes
    end
  end

  devise_for :users,
    path: '',
    path_names: {
      sign_up: '',
      sign_in: 'login',
      sign_out: 'logout',
      registration: 'signup'
    },
    controllers: {
      registrations: 'users/registrations',
      sessions: 'users/sessions',
      passwords: 'users/passwords'
    }
  devise_scope :user do
    get 'signup', to: 'users/registrations#new'
    get 'login', to: 'users/sessions#new'
    get 'logout', to: 'users/sessions#destroy'
  end

  resources :posts, only: [:create, :destroy] do
    collection do
      get :search
    end
  end

  resources :relationships, only: [:create, :destroy]
  resources :favorites, only: [:create, :destroy]

end

解決方法

「２箇所定義とかしてないのにな〜」とか考えていましたが、ふと一番上に行が追加されていることに気づきましたｗ
devise_for :users の行を削除することで解決。

routes.rb

Rails.application.routes.draw do
  root to: 'toppages#index'

  resources :users, only: [:index, :show, :edit, :update, :destroy] do
    member do
      get :followings
      get :followers
      get :likes
    end
  end

  devise_for :users,
    path: '',
    path_names: {
      sign_up: '',
      sign_in: 'login',
      sign_out: 'logout',
      registration: 'signup'
    },
    controllers: {
      registrations: 'users/registrations',
      sessions: 'users/sessions',
      passwords: 'users/passwords'
    }
  devise_scope :user do
    get 'signup', to: 'users/registrations#new'
    get 'login', to: 'users/sessions#new'
    get 'logout', to: 'users/sessions#destroy'
  end

  resources :posts, only: [:create, :destroy] do
    collection do
      get :search
    end
  end

  resources :relationships, only: [:create, :destroy]
  resources :favorites, only: [:create, :destroy]

end

無事、起動することができました！

最後に

already in use系のエラーは、今回と同様に同じルーティングが既に設定されていると考えて良さそうですね。
（そのままですが）
今後同じエラーが出た際は、サクッと解決できそう。