v3 shows duplicate results for unused (-u) about eslint-find-rules HOT 3 CLOSED

I.e. [...new Set(arr)]

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First of all, thanks for providing that repo as a test case!

Strange thing is: the getRules method exposed by ESLint includes rules of plugins on ESLint ^3.12¹⁾, but does not on ESLint ^4.0. So we duplicate the plugins' rules on ESLint ^3.12 when concatenating only pluginsRules and (what we think are only) coreRules.

This means, we have to get rid of the plugins' rules erroneously (?) included in the getRules methods response, when running using ESLint ^3.12.

We could exclude every item from coreRules that matches ^[^\/]+\/ (starting at least one character not being a slash, followed by a slash) or that is included in pluginRules (using our array-diff utility function).
Additionally we could do this only when require('eslint').linter.version matches ^3.12 according to semver, which I would definitely prefer.

1. This only happens within the context of eslint-find-rules. When only running the Node repl, requireing ESLint and calling getRules, the plugins' rules are not included.
   Like this
   console.log(Array.from(require('eslint').linter.getRules().keys()).join('\n'));

from eslint-find-rules.

Seems like just deduping unconditionally makes the most sense?

from eslint-find-rules.