accepts_nested_attributes_for is not working for uniqueness

[Rajeev Kannav Sharma]

class User < ActiveRecord::Base

  accepts_nested_attributes_for :emails,      :allow_destroy => true,   :reject_if => proc { |attributes| attributes['address'].blank? }

end

class Email < ActiveRecord::Base

  belongs_to :user

  validates_presence_of :address

  validates_email_format_of :address

  validates_uniqueness_of :address, :scope=>'user_id'

end

when i am saving multiple same emails at one time it is saving , but it should not

Please suggest what i am missing 

Thanks

Rajeev sharma