Example double error output for https://github.com/rails/rails/issues/10336

app_models_guaranty.rb

class Guaranty < ActiveRecord::Base
  belongs_to :product
  accepts_nested_attributes_for :product
  validates :name, presence: true
end

app_models_product.rb

class Product < ActiveRecord::Base
  has_many :guaranties
  accepts_nested_attributes_for :guaranties
  validates :name, presence: true
end

Console Output

$ rails new issue10336
$ cd issue10336
$ rails g model guaranty name:string product_id:integer
$ rails g model product name:string
$ rake db:migrate

$ rails console
Loading development environment (Rails 4.0.0.rc1)

1.9.3-p194 :001 > g = Guaranty.new
 => #<Guaranty id: nil, name: nil, product_id: nil, created_at: nil, updated_at: nil>

1.9.3-p194 :002 > p = Product.new
 => #<Product id: nil, name: nil, created_at: nil, updated_at: nil>

1.9.3-p194 :003 > p.guaranties << g
 => #<ActiveRecord::Associations::CollectionProxy [#<Guaranty id: nil, name: nil, 
      product_id: nil, created_at: nil, updated_at: nil>]>

1.9.3-p194 :006 > g.product = p
 => #<Product id: nil, name: nil, created_at: nil, updated_at: nil>

1.9.3-p194 :007 > p.valid?
 => false

1.9.3-p194 :008 > p.errors.full_messages
 => ["Name can't be blank", "Name can't be blank", 
     "Guaranties product name can't be blank", "Guaranties name can't be blank"]