Add new Bundler/DuplicatedGroup cop

[OwlKing]

New cop validating each gem group (or a set of groups) is only defined once.

# bad
group :development do
  gem 'rubocop'
end

group :development do
  gem 'rubocop-rails'
end

# bad
group :development, :test do
  gem 'rubocop'
end

group :test, :development do
  gem 'rspec'
end

# good
group :development do
  gem 'rubocop'
end

group :development, :test do
  gem 'rspec'
end

# good
gem 'rubocop', groups: [:development, :test]
gem 'rspec', groups: [:development, :test]

This has been suggested many years ago here #3600 but I couldn't find any attempts at implementing it.

---

Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.

[bbatsov]

Given the examples that's not exactly true, so you might want to expand this description.

[bbatsov]

It'd be nice to explain why this is bad and the next example is good, as it's not obvious to me.

[bbatsov]

(to be clear - it took me a moment to see that the names were the same, but in different order)

[matthewhively]

@bbatsov
Forgive the necro post.
I'm just curious, why was this cop added as Enabled: true? shouldn't it have been pending?

[bbatsov]

@matthewhively An oversight on my part. :-) At any rate, it's more or less a Lint cop, so I'm guessing most people would have enabled it anyways.