Add duplicate package checking in the webpack build.

dev_mode/package.json

@@ -102,6 +102,7 @@
   "devDependencies": {
     "@jupyterlab/buildutils": "^0.7.2",
     "css-loader": "~0.28.7",
+    "duplicate-package-checker-webpack-plugin": "^3.0.0",
     "file-loader": "~1.1.11",
     "fs-extra": "~4.0.2",
     "glob": "~7.1.2",

dev_mode/webpack.config.js

@@ -8,6 +8,7 @@ var fs = require('fs-extra');
 var Handlebars = require('handlebars');
 var HtmlWebpackPlugin = require('html-webpack-plugin');
 var webpack = require('webpack');
+var DuplicatePackageCheckerPlugin = require("duplicate-package-checker-webpack-plugin");
 
 var Build = require('@jupyterlab/buildutils').Build;
 var package_data = require('./package.json');
@@ -177,6 +178,9 @@ module.exports = {
   bail: true,
   devtool: 'source-map',
   plugins: [
+    new DuplicatePackageCheckerPlugin({
+      verbose: true
+    }),
     new HtmlWebpackPlugin({
       template: path.join('templates', 'template.html'),
       title: jlab.name || 'JupyterLab'

yarn.lock

@@ -2742,6 +2742,15 @@ duplexify@^3.4.2, duplexify@^3.5.3:
     readable-stream "^2.0.0"
     stream-shift "^1.0.0"
 
+duplicate-package-checker-webpack-plugin@^3.0.0:
+  version "3.0.0"
+  resolved "https://registry.npmjs.org/duplicate-package-checker-webpack-plugin/-/duplicate-package-checker-webpack-plugin-3.0.0.tgz#78bb89e625fa7cf8c2a59c53f62b495fda9ba287"
+  dependencies:
+    chalk "^2.3.0"
+    find-root "^1.0.0"
+    lodash "^4.17.4"
+    semver "^5.4.1"
+
 ecc-jsbn@~0.1.1:
   version "0.1.1"
   resolved "https://registry.npmjs.org/ecc-jsbn/-/ecc-jsbn-0.1.1.tgz#0fc73a9ed5f0d53c38193398523ef7e543777505"
@@ -3341,6 +3350,10 @@ find-cache-dir@^1.0.0:
     make-dir "^1.0.0"
     pkg-dir "^2.0.0"
 
+find-root@^1.0.0:
+  version "1.1.0"
+  resolved "https://registry.npmjs.org/find-root/-/find-root-1.1.0.tgz#abcfc8ba76f708c42a97b3d685b7e9450bfb9ce4"
+
 find-up@^1.0.0:
   version "1.1.2"
   resolved "https://registry.npmjs.org/find-up/-/find-up-1.1.2.tgz#6b2e9822b1a2ce0a60ab64d610eccad53cb24d0f"