Add new Gemspec/DuplicatedAssignment cop

[koic]

Feature

An attribute assignment method calls should be listed only once in a gemspec.

Assigning to an attribute with the same name using spec.foo = will be an unintended usage. On the other hand, duplication of methods such as spec.requirements, spec.add_runtime_dependency and others are permitted because it is the intended use of appending values.

Bad case

Gem::Specification.new do |spec|
  spec.name = 'rubocop'
  spec.name = 'rubocop2'
end

Good case

Gem::Specification.new do |spec|
  spec.name = 'rubocop'
end

Gem::Specification.new do |spec|
  spec.requirements << 'libmagick, v6.0'
  spec.requirements << 'A good graphics
end

Gem::Specification.new do |spec|
  spec.add_runtime_dependency('parallel', '~> 1.10')
  spec.add_runtime_dependency('parser', '>= 2.3.3.1', '< 3.0')
end

How to use

% cat foo.gemspec
# frozen_string_literal: true

Gem::Specification.new do |spec|
  spec.name = 'rubocop'
  spec.name = 'rubocop2'
end

% rubocop foo.gemspec
Inspecting 1 file
C

Offenses:

/tmp/foo.gemspec:5:3: C: Gemspec/DuplicatedAssignment: name= method
calls already given on line 4 of the gemspec.
  spec.name = 'rubocop2'
  ^^^^^^^^^^^^^^^^^^^^^^

1 file inspected, 1 offense detected

The implementation approach is based on Bundler/DuplicatedGem.
https://github.com/bbatsov/rubocop/blob/v0.51.0/lib/rubocop/cop/bundler/duplicated_gem.rb

Other Information

RubyGems Specification Reference
http://guides.rubygems.org/specification-reference/

---

Before submitting the PR make sure the following are checked:

• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Used the same coding conventions as the rest of the project.
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• All tests(rake spec) are passing.
• The new code doesn't generate RuboCop offenses that are checked by rake internal_investigation.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Updated cop documentation with rake generate_cops_documentation (required only when you've added a new cop or changed the configuration/documentation of an existing cop).

[bbatsov]

👍