Docs: Don't warn if we find the same entry twice.

[tmeasday]

Closes #21373

What I did

Don't warn if we find the same entry (by import path) twice; this can mean it's specified twice. E.g.

    '../src/docs/Introduction.mdx',
    '../src/docs/*.mdx',

How to test

Update a sandbox to include:

"stories": ["../src/stories/Introduction.mdx", "../src/**/*.mdx", "../src/**/*.stories.@(js|jsx|ts|tsx)", {`

Double check it doesn't warn.

Checklist

• Make sure your changes are tested (stories and/or unit, integration, or end-to-end tests)
• Make sure to add/update documentation regarding your changes
• If you are deprecating/removing a feature, make sure to update
  MIGRATION.MD

Maintainers

• If this PR should be tested against many or all sandboxes,
  make sure to add the ci:merged or ci:daily GH label to it.
• Make sure this PR contains one of the labels below.

["cleanup", "BREAKING CHANGE", "feature request", "bug", "documentation", "maintenance", "dependencies", "other"]