Fix an incorrect autocorrect for Style/SpecialGlobalVars

[koic]

This PR fixes the following incorrect autocorrect for Style/SpecialGlobalVars when global variable as Perl name is used multiple times.

Example:

% cat example.rb
$*[0]
$*[1]

% bundle exec rubocop --only Style/SpecialGlobalVars -A
Inspecting 3 files
.C.

Offenses:

example.rb:1:1: C: [Corrected] Style/SpecialGlobalVars: Prefer $ARGV
from the stdlib 'English' module (don't forget to require it) or ARGV
over $*.
$*[0]
^^
example.rb:2:1: C: [Corrected] Style/SpecialGlobalVars: Prefer $ARGV
from the stdlib 'English' module (don't forget to require it) or ARGV
over $*.
$*[1]
^^

3 files inspected, 2 offenses detected, 2 offenses corrected

This auto-correction will add require 'English' twice:

% cat example.rb
require 'English'
require 'English'
$ARGV[0]
$ARGV[1]

This PR ensures that require 'English' is added only once:

% cat example.rb
require 'English'
$ARGV[0]
$ARGV[1]

---

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!