Warn about duplicate dependencies

[darrenscerri]

Add duplicate-package-checker-webpack-plugin as proposed here: #1844.

[facebook-github-bot]

Thank you for your pull request and welcome to our community. We require contributors to sign our Contributor License Agreement, and we don't seem to have you on file. In order for us to review and merge your code, please sign up at https://code.facebook.com/cla - and if you have received this in error or have any questions, please drop us a line at cla@fb.com. Thanks!

If you are contributing on behalf of someone else (eg your employer): the individual CLA is not sufficient - use https://developers.facebook.com/opensource/cla?type=company instead. Contact cla@fb.com if you have any questions.

[facebook-github-bot]

Thank you for signing our Contributor License Agreement. We can now accept your code for this (and any) Facebook open source project. Thanks!

[Timer]

Since this is just a warning, I don't think we have to consider it breaking. /cc @gaearon

[darrenscerri]

I have two issues with this pull request, maybe someone can help me out.

I noticed that warnings are not being displayed when building. The solution would be to display any warnings from react-scripts/scripts/build.js.

The second issue is that the tests for kitchensink are failing since babel-runtime is duplicated in the bundle and the tests fail on compilation warnings:

Warning in duplicate-package-checker:
  <babel-runtime>
    6.20.0 ./~/react-dev-utils/~/babel-runtime from ./~/react-dev-utils/~/stack-frame-resolver/lib/index.js
    6.23.0 /Users/Darren/projects/create-react-app/packages/babel-preset-react-app/~/babel-runtime from ./src/features/syntax/ArraySpread.js

After some digging, it appears that in the kitchensink, two different babel-runtime versions are being included, one in the main chunk and one in the features/syntax/ArraySpread.js chunk.

Any suggestions?

[Timer]

You can bump stack-frame-resolver to 0.1.5 (which relaxes the dep to ^6.20.0) or switch this PR to the 0.9.x branch, both should fix the problem (with duplicate deps).
This PR is already doing great things! 😅

[darrenscerri]

Created new PR to the 0.9.x branch #1866 . Closing this PR.

[darrenscerri]

@Timer can you add tags and milestone to the other PR please?