# Using a service to remove duplication
class UsersController < ApplicationController
  def create
    @user = User.new params[:user]
    UserService.signup @user
  end
end

class Admin::UsersController < AdminController
  def create
    @user = User.new params[:user]
    UserService.signup(@user)
  end
end

class UserService
  def self.signup(user)
    UserMailer.deliver_welcome(user) if user.save
  end
end