Add error message when user tries to follow their own account (#31910)

app/controllers/api/v1/accounts_controller.rb

@@ -16,6 +16,7 @@ class Api::V1::AccountsController < Api::BaseController
   before_action :check_account_confirmation, except: [:index, :create]
   before_action :check_enabled_registrations, only: [:create]
   before_action :check_accounts_limit, only: [:index]
+  before_action :check_following_self, only: [:follow]
 
   skip_before_action :require_authenticated_user!, only: :create
 
@@ -101,6 +102,10 @@ class Api::V1::AccountsController < Api::BaseController
     raise(Mastodon::ValidationError) if account_ids.size > DEFAULT_ACCOUNTS_LIMIT
   end
 
+  def check_following_self
+    render json: { error: I18n.t('accounts.self_follow_error') }, status: 403 if current_user.account.id == @account.id
+  end
+
   def relationships(**options)
     AccountRelationshipsPresenter.new([@account], current_user.account_id, **options)
   end