Add Errors Once

example.rb

message = 'This is some error message'
errors.add(:attribute, message) unless errors.added?(:attribute, message)

# or should we create an add_once on errors. Can be done as a monkey patch but a core contribution would be better
def ActiveRecord::Errors
  def add_once(attribute, message)
    add(attribute, message) unless added?(attribute, message)
  end
end

errors.add_once(:attribute, 'this is some error message')