rails generate devise User error when migrating - ruby-on-rails

Question

I'm a bit new in Rails world and I try to add a new field in devise schema.
I found this :
rails generate model NAME [field[:type][:index] field[:type]
and tried to apply the command :
rails generate devise User linkedin:string
The process seemed correct :
invoke active_record
create db/migrate/20130902085306_add_devise_to_users.rb
insert app/models/user.rb
route devise_for :users
But when I launch a db:migrate it occures an error :
PG::Error: ERROR: column "email" of relation "users" already exists
What did I do wrong ? why does it say (and is it related) email is wrong while it was ok before ?
Thanks a lot !
Here is the migration file result :
class AddDeviseToUsers < ActiveRecord::Migration
def self.up
change_table(:users) do |t|
## Database authenticatable
t.string :email, :null => false, :default => ""
t.string :encrypted_password, :null => false, :default => ""
## Recoverable
t.string :reset_password_token
t.datetime :reset_password_sent_at
## Rememberable
t.datetime :remember_created_at
## Trackable
t.integer :sign_in_count, :default => 0
t.datetime :current_sign_in_at
t.datetime :last_sign_in_at
t.string :current_sign_in_ip
t.string :last_sign_in_ip
## Confirmable
# t.string :confirmation_token
# t.datetime :confirmed_at
# t.datetime :confirmation_sent_at
# t.string :unconfirmed_email # Only if using reconfirmable
## Lockable
# t.integer :failed_attempts, :default => 0 # Only if lock strategy is :failed_attempts
# t.string :unlock_token # Only if unlock strategy is :email or :both
# t.datetime :locked_at
## Token authenticatable
# t.string :authentication_token
t.string :linkedin
# Uncomment below if timestamps were not included in your original model.
# t.timestamps
end
add_index :users, :email, :unique => true
add_index :users, :reset_password_token, :unique => true
# add_index :users, :confirmation_token, :unique => true
# add_index :users, :unlock_token, :unique => true
# add_index :users, :authentication_token, :unique => true
end
def self.down
# By default, we don't want to make any assumption about how to roll back a migration when your
# model already existed. Please edit below which fields you would like to remove in this migration.
raise ActiveRecord::IrreversibleMigration
end
end

Answer 1

Does your users table already have email field? It seems there is, you have to remove creating an email field again by removing it from migration and make sure it is not referenced twice in user.rb model
db/migrate/20130902085306_add_devise_to_users.rb

Answer 2

Ok!
I've found what was wrong : I created a new table instead of updating existing one.
So the good task was :
rails g migration add_columnLinkedin_to_users
Then adding in the new created migration file :
change_table :users do |t|
t.string :linkedin
end
And db:migrate was a success !
Thanks for your helps !