Server side mixpanel analytics implementation in ruby. There are other simple devise controllers that are overridden omitted from gist.

analytic.rb

# http://blog.mixcel.io/10-ways-to-get-mixpanel-right-the-first-time-yHNlbz-wbcZz7E7PWMXvRg
class Analytic
  module Mixpanelable
    extend ActiveSupport::Concern

    private

    def mp_track(event_name, options = {})
      mp_track_for_user(current_user, event_name, options)
    end

    def mp_track_charge(amount, options = {})
      return unless token.present?

      options.merge! team: @organization.name if @organization.present?
      options.merge! ip: current_user.current_sign_in_ip.to_s
      mixpanel.people.track_charge(current_user.id, amount.to_f, options)
    end

    def mp_track_for_user(user, event_name, options = {})
      return unless token.present?

      options.merge! team: @organization.name if @organization.present?
      options.merge! ip: user.current_sign_in_ip.to_s
      mixpanel.track(user.id, event_name, options)
    end

    def set_mp_cookie_information
      # In your case, the cookies will be namespaced differently
      # so remember to use your own namespace.
      if (mp_cookies = cookies[namespace])
        @mp_properties = JSON.parse(mp_cookies)
      end
    end

    def identify_on_mixpanel(user)
      return unless token.present?
      raise ArgumentError, "User cannot be blank when identifying on mixpanel" if user.blank?
      Rails.logger.info "## Identifying User #{user.id} with Mixpanel"

      # attributes is existing Mixpanel Params that are cookie'd by
      # Mixpanel Javascript
      attributes = safely_retrieve_attributes
      mixpanel_params = {
        '$email': user.email,
        '$created': user.created_at.as_json,
        '$first_name': user.first_name,
        '$last_name': user.last_name,
        '$ip': user.current_sign_in_ip.to_s,
        'user_id': user.id,
        'Teams': user.organizations.map(&:name)
      }.merge(attributes)

      distinct_id = mixpanel_params['distinct_id']

      # Alias the User with the old distinct ID
      mixpanel.alias user.id, distinct_id if distinct_id.present?

      # Set user properties
      Rails.logger.info "## Current Mixpanel User IP: #{user.current_sign_in_ip.to_s}"
      mixpanel.people.set user.id, mixpanel_params, user.current_sign_in_ip.to_s

      mp_track_for_user user, "Identified"
    end

    def mixpanel
      @mixpanel ||= Mixpanel::Tracker.new(token)
    end

    def token
      !Rails.env.test? && ENV['MIXPANEL_TOKEN']
    end

    def namespace
      "mp_#{token}_mixpanel"
    end

    def safely_retrieve_attributes
      JSON.parse(cookies[namespace]) || {}
    rescue JSON::ParserError => e
      Honeybadger.notify(
        :error_class   => "Mixpanel Analytics Cookie Retrieval",
        :error_message => e.message,
        :parameters    => { cookie: cookies[namespace] }
      )
      {}
    end
  end
end

registrations_controller.rb

class DeviseOverride::RegistrationsController < Devise::RegistrationsController
  include Analytic::Mixpanelable
  before_filter :set_mp_cookie_information

  def create
    super do |resource|
      Rails.logger.info "## Inside DeviseOverride::RegistrationsController"
      if resource.persisted?
        resource.mp_properties = @mp_properties
        resource.save
        identify_on_mixpanel(resource)
      end
    end
  end

  private

  def sign_up_params
    params.require(:user).permit(:first_name, :last_name, :email, :password, :password_confirmation).merge(role: :user)
  end

  def account_update_params
    params.require(:user).permit(:first_name, :last_name, :email, :password, :password_confirmation, :current_password)
  end
end

sessions_controller.rb

class DeviseOverride::SessionsController < Devise::SessionsController
  include Analytic::Mixpanelable
  before_filter :set_mp_cookie_information

  def create
    super do |resource|
      Rails.logger.info "## Inside DeviseOverride::SessionsController"
      identify_on_mixpanel(resource) if resource.persisted?
    end
  end

  def update
    super do |resource|
      identify_on_mixpanel(resource) if resource.persisted?
    end
  end
end

as per MP docs, alias should be called only once after signup, but in your code it's called each time user signin. Am I misunderstanding something?