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)