Extract helper module for duplicate detection

[Darhazer]

#994 would be the 5th cop that deals with repeated usage, and all the cops share similar structure for the detection: node.children.select { ... }.group_by { .... }.values.reject(&:one?).flat_map { repeated_lines }

It's time to extract a module for this and DRY the code

[bquorning]

The five cops are:

1. RuboCop::Cop::RSpec::RepeatedExample
2. RuboCop::Cop::RSpec::RepeatedExampleGroupBody
3. RuboCop::Cop::RSpec::RepeatedExampleGroupDescription
4. RuboCop::Cop::RSpec::RepeatedIncludeExample
5. RuboCop::Cop::RSpec::ScatteredSetup