Add cop for checking duplicate assingments to ignored_columns

[fsateler]

Overwriting previous assignments is usually a mistake, since it will
un-ignore the first set of columns

---

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).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• 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.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Run bundle exec rake default. It executes all tests and RuboCop for itself, and generates the documentation.
• [] If this is a new cop, consider making a corresponding update to the Rails Style Guide. (I don't think this is needed)

[koic]

I think the following should be technically permitted.

expect_no_offenses(<<~RUBY)
  class User < ActiveRecord::Base
    if condition
      self.ignored_columns = [:one]
    else
      self.ignored_columns = [:two]
    end
  end
RUBY

[fsateler]

I think the following should be technically permitted.

Oh, good point. Indeed it should. Any hints on how to approach that? A cop that does something similar to copy get inspiration?

[koic]

I'm not sure, but Style/IdenticalConditionalBranches may be a hint to check that different branches have the same statement.
https://github.com/rubocop/rubocop/blob/master/lib/rubocop/cop/style/identical_conditional_branches.rb

[pirj]

Looks good, but I believe we can make it stricter with no negative impact.

[fsateler]

I couldn't make it work with conditional branches :(. Will that be a blocker?

[fsateler]

I added a PR for rails style guide rubocop/rails-style-guide#301

[kkitadate]

@fsateler @koic @pirj Waiting for this PR to be merged. What's Blocker? (Though I think #514 (comment) is related)

[pirj]

Can you please force-push to trigger CI, @kkitadate ?

[kkitadate]

Can you please force-push to trigger CI, @kkitadate ?

Sure. I will open another PR as I can't push to this branch.

[kkitadate]

@pirj I opened #771. It also seems to have passed CI.

[koic]

@fsateler I'm going close this PR because #771 has been merged. I'm sorry about should have proceeded this one. So, I've added your account name to the changelog entry as the original author.
7a434e4

Thank you.

[fsateler]

@koic thanks for the kind words. I'm glad someone else made it work faster. As long as it happened I'm happy :)

Thank you for maintaining rubocop-rails!