If you need to make sure that only one Devise user class is logged in and that users can't register as another class if they are already logged in, you need to pre-check their status. To do that use these following snippets.

registrations_controller.rb

# Create this new controller for registrations to check that they're not already registered

class RegistrationsController < Devise::RegistrationsController

	def new
		if (user_signed_in? || admin_signed_in?)
			redirect_to '/', alert: "You cannot register two roles at the same time!"
		else
			super
		end
	end
end

routes.rb

# Put :controllers => {:sessions => "sessions", :registrations => "registrations"} after all devise_for

YourApp::Application.routes.draw do
  
  devise_for :users, :controllers => {:sessions => "sessions", :registrations => "registrations"}
  devise_for :admins, :controllers => {:sessions => "sessions", :registrations => "registrations"}
  
end  

sessions_controller.rb

# Create this new controller for sessions to check that they're not already logged in

class SessionsController < Devise::SessionsController

	def new
		if (user_signed_in? || admin_signed_in?)
			redirect_to '/', alert: "You can only sign into a single user!"
		else
			super
		end
	end
end

You can also redirect to :back instead of '/'