"Only one instance of babel-polyfill is allowed" error
If the culprit is HtmlWebpackPlugin, you need to add the option inject: false when instancing the plugin. Certain configurations without this option causes your built javascript code to be loaded twice.
Idempotent Babel Polyfill can be imported multiple times
Install from NPM
npm install --save idempotent-babel-polyfillThen import it
import 'idempotent-babel-polyfill';
Only one instance of babel-polyfill is allowed usually appears if the order of files being wrapped isn't correct when making use of CommonsChunkPlugin or HtmlWebpackPlugin.
for HtmlWebpackPlugin you can manually sort your files with chunksSortMode
Using "webpack": "^1.14.0":
new HtmlWebpackPlugin({ ... chunksSortMode: 'dependency', ...}),