What's the difference between Rails dup and clone methods?

I need to know the difference between Rails dup and clone methods, because dup duplicates the id attribute and clone not duplicates the id attribute

juan:~/alhambra$ rails c
Loading development environment (Rails 3.0.1)
1.9.3-p551 :001 > @user=User.last
 => #<User id: 2, email: "ferbad12@hotmail.com", encrypted_password: "$2a$10$/Fsz8DZ9PQbReTU1.wyxS.m3IOxZSV3siKDrrtUJdupz...", password_salt: "$2a$10$/Fsz8DZ9PQbReTU1.wyxS.", reset_password_token: nil, remember_token: nil, remember_created_at: nil, sign_in_count: 1, current_sign_in_at: "2015-05-06 23:34:20", last_sign_in_at: "2015-05-06 23:34:20", current_sign_in_ip: "127.0.0.1", last_sign_in_ip: "127.0.0.1", created_at: "2015-05-06 23:33:37", updated_at: "2015-05-06 23:34:20"> 
1.9.3-p551 :002 > @user.clone
 => #<User id: nil, email: "ferbad12@hotmail.com", encrypted_password: "$2a$10$/Fsz8DZ9PQbReTU1.wyxS.m3IOxZSV3siKDrrtUJdupz...", password_salt: "$2a$10$/Fsz8DZ9PQbReTU1.wyxS.", reset_password_token: nil, remember_token: nil, remember_created_at: nil, sign_in_count: 1, current_sign_in_at: "2015-05-06 23:34:20", last_sign_in_at: "2015-05-06 23:34:20", current_sign_in_ip: "127.0.0.1", last_sign_in_ip: "127.0.0.1", created_at: "2015-05-06 23:33:37", updated_at: "2015-05-06 23:34:20"> 
1.9.3-p551 :003 > @user.dup
 => #<User id: 2, email: "ferbad12@hotmail.com", encrypted_password: "$2a$10$/Fsz8DZ9PQbReTU1.wyxS.m3IOxZSV3siKDrrtUJdupz...", password_salt: "$2a$10$/Fsz8DZ9PQbReTU1.wyxS.", reset_password_token: nil, remember_token: nil, remember_created_at: nil, sign_in_count: 1, current_sign_in_at: "2015-05-06 23:34:20", last_sign_in_at: "2015-05-06 23:34:20", current_sign_in_ip: "127.0.0.1", last_sign_in_ip: "127.0.0.1", created_at: "2015-05-06 23:33:37", updated_at: "2015-05-06 23:34:20">