Add transform-duplicate-keys mapping

[existentialism]

Thanks to @Timer, it looks like we missed mapping this transform causing output to be different than preset-latest.

I double checked that the rest of the plugins are mapped.

Ref: facebook/create-react-app#1742

[Timer]

You're a saint. Thanks!

[yavorsky]

👍

[hzoo]

Oh interesting 😮 . Do we want a test for the output as well?

[yavorsky]

@hzoo We are not testing all plugins (just opts for regenerator and transform-polyfill-require-plugin), so I think it's not necessarily. Updating stdout.txt should be ok. Or we could add tests to cover all or most of plugins.
Maybe let's test matching preset-latest without targets?

[Timer]

Anything I can do to help?

[existentialism]

Added a quick and dirty sanity check test, it doesn't hurt to have these until we move to more comprehensive tests on Browserstack or some other means.