Fix duplicate extension cop versions when rubocop -V

[koic]

Follow #9349 (comment).

This PR fixes duplicate extension cop versions when rubocop -V.

The following is an example:

# ext.yml
require:
  - rubocop-rspec

# .rubocop.yml
inherit_from: ext.yml
require:
  - rubocop-performance
  - rubocop-rspec

## Before

If there are duplicate requires, extension cop display will also be duplicated.
Duplicate requires will be displayed in duplicate.

```console
% rubocop -V
1.8.0 (using Parser 3.0.0.0, rubocop-ast 1.4.0, running on ruby 2.7.2 x86_64-darwin19)
  - rubocop-performance 1.9.2
  - rubocop-rspec 2.0.0
  - rubocop-rspec 2.0.0

After

Even if there are duplicate requires, they will only be displayed once per extension cop.

% rubocop -V
1.8.0 (using Parser 3.0.0.0, rubocop-ast 1.4.0, running on ruby 2.7.2 x86_64-darwin19)
  - rubocop-performance 1.9.2
  - rubocop-rspec 2.0.0

Replace this text with a summary of the changes in your PR.
The more detailed you are, the better.

---

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]

Thanks!