title: "Only one instance of babel-polyfill is allowed" error
body: <p>I need help diagnosing and fixing this error:</p>
  
  <pre><code>"Error: only one instance of babel-polyfill is allowed"
  </code></pre>
  
  <p>I have the following in my package.json:</p>
  
  <pre><code>"devDependencies": {
      "babel-core": "^6.23.1",
      "babel-jest": "^19.0.0",
      "babel-loader": "^6.3.2",
      "babel-plugin-transform-object-rest-spread": "^6.23.0",
      "babel-preset-es2015": "^6.22.0",
      "babel-preset-react": "^6.23.0" ...
  
  "dependencies": {
      "babel-polyfill": "^6.23.0" ...
  </code></pre>
  
  <p>And this and this entry line in my webpack config: </p>
  
  <pre><code>entry: ["babel-polyfill", path.resolve(APP_PATH, 'index')],
  ...
  module: {
   rules: [
    {
      test: /\.js$/,
      exclude: /node_modules/,
      loader: 'babel-loader',
      query: {
        // specify that we will be dealing with React code
        presets: ['react', 'es2015']
      }
    }
  ]}
  </code></pre>