http://akkartik.name/blog/2011-07-15-01-27-52-soc

after.rb

def create
  @user = User.find_by_email(params[:user][:email]) || User.create(params[:user])
  return render(:action => 'new') if @user.new_record? # redisplay form
  log('Created', @user)
  session[:user] = @user
end

before.rb

def create
  @user = User.find_by_email([:user][:email])
  unless @user
    @user = User.new(params[:user])
    if @user.save # successfully saved
      log('Created', @user)
      session[:user] = @user
    else
      render(:action => 'new') # redisplay form with @user's errors
    end
  end
end