No complaint even though two versions of aws-amplify in build

[ceich]

I have added this plugin to devDependencies, and here is my webpack.config.js:

var DuplicatePackageCheckerPlugin = require("duplicate-package-checker-webpack-plugin");

module.exports = {
  plugins: [new DuplicatePackageCheckerPlugin({
    verbose: true,
  })]
};

Problem: neither yarn build nor yarn start complain about this duplicate package:

$ grep aws-amplify yarn.lock
aws-amplify-react@^0.1.52:
  resolved "https://registry.yarnpkg.com/aws-amplify-react/-/aws-amplify-react-0.1.52.tgz#0080a10ad14447a4a33ebe3855917c98ed009148"
    aws-amplify "^0.4.6"
aws-amplify@^0.3.4:
  resolved "https://registry.yarnpkg.com/aws-amplify/-/aws-amplify-0.3.4.tgz#fc7e9398a51f6d071f3e3dd50017509b67efedfc"
aws-amplify@^0.4.6:
  resolved "https://registry.yarnpkg.com/aws-amplify/-/aws-amplify-0.4.6.tgz#aa6485876ca37c75b600883be52924be78b2840f"

I am a newbie with webpack and yarn, so I am probably overlooking something obvious. Any pointers would be gratefully appreciated.