Duplicate field names when using ActiveHash::Enum

Environment

Ruby version: 3.3.4
ActiveHash version: 3.3.1

Description of the issue

When using ActiveHash::Enum in combination with ActiveHash::Base, the field_names method returns duplicate field names. This occurs only when enum_accessor is used.

Steps to reproduce

1. Create an ActiveHash::Base class with some data
2. Include ActiveHash::Enum
3. Use enum_accessor
4. Call .field_names on the class

Example code

class Sport < ActiveHash::Base
  self.data = [
    {
      key: "tennis",
      category: "ball",
    },
    {
      key: "soccer",
      category: "ball",
    },
  ]

  include ActiveHash::Enum
  enum_accessor :key
end

Sport.field_names
# => [:key, :category, :key, :category]

Expected behavior

The field_names method should return unique field names:

Sport.field_names
# => [:key, :category]

Additional information

This issue does not occur when ActiveHash::Enum is included without using enum_accessor:

class Sport < ActiveHash::Base
  self.data = [
    {
      key: "tennis",
      category: "ball",
    },
    {
      key: "soccer",
      category: "ball",
    },
  ]

  include ActiveHash::Enum
end

Sport.field_names
# => [:key, :category]

Reproduce the example with ActiveHash::Base, ActiveHash::Enum, and enum_accessor, then inspect the field_names and enum_accessor entry points. Compare the returned fields with and without enum_accessor; done means Sport.field_names returns [:key, :category] without duplicates while the existing behavior remains intact.