Extract helper module to detect repeated nodes

[Darhazer]

Fixes #996

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Before submitting the PR make sure the following are checked:

• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Updated documentation.
• Added an entry to the CHANGELOG.md if the new code introduces user-observable changes.
• The build (bundle exec rake) passes (be sure to run this locally, since it may produce updated documentation that you will need to commit).

[r7kamura]

Wouldn't it be simpler to just use a block instead of passing a Proc instance as an argument?

Suggested change

	def find_repeated_groups(items, key_proc:)
	items
	.group_by(&key_proc)
	.values
	.reject(&:one?)
	end
	def find_repeated_groups(items, &block)
	items
	.group_by(&block)
	.values
	.reject(&:one?)
	end

[bquorning]

I actually found the code a bit confusing, when I tried rewriting it with an implicit block just now. If we do do that however, could we rename the block argument from &block to e.g. &signature_proc?

[r7kamura]

That makes sense.

Without keyword arguments to label things, it can be hard to tell what's going on from the caller’s side. In that case, the original way of passing a Proc via keyword arguments definitely has its perks. Alternatively, it might improve things somewhat to change the method name to something like find_repeated_groups_by, which clearly implies that grouping logic is expected.

Since both approaches seem to have their pros and cons, I felt that leaving it as it is would also be fine 👌

[r7kamura]

As a general policy, I agree with this change 👍