add confirmable to devise

Rails3.1.3
devise (2.0.4)

I have set up a table: users using devise. But it gives an error

  undefined local variable or method `unconfirmed_email' for
#<User:0x00000102231218>

So I generated migration file and edited

class AddConfirmableToDevise < ActiveRecord::Migration
  def change
    change_table(:users) do |t|
      t.confirmable
    end
  end
end

$rake db:migrate

command produces

== AddConfirmableToDevise: migrating
-- change_table(:users)

[DEVISE] You are using t.database_authenticatable and others in your
migration and this feature is d
eprecated. Please simply use Rails helpers instead as mentioned here:
https://github.com/plataformatec/devise/wiki/How-To:-Upgrade-to-Devise-2.0-migration-schema-style

rake aborted!
An error has occurred, this and all later migrations canceled:

SQLite3::SQLException: duplicate column name: confirmation_token: ALTER
TABLE "users" ADD "confirmat
ion_token" varchar(255)