Warn about duplicate dependencies

packages/react-scripts/config/webpack.config.dev.js

@@ -17,6 +17,7 @@ const InterpolateHtmlPlugin = require('react-dev-utils/InterpolateHtmlPlugin');
 const WatchMissingNodeModulesPlugin = require('react-dev-utils/WatchMissingNodeModulesPlugin');
 const eslintFormatter = require('react-dev-utils/eslintFormatter');
 const ModuleScopePlugin = require('react-dev-utils/ModuleScopePlugin');
+const DuplicatePackageCheckerPlugin = require('duplicate-package-checker-webpack-plugin');
 const getClientEnvironment = require('./env');
 const paths = require('./paths');
 
@@ -268,6 +269,10 @@ module.exports = {
     // https://github.com/jmblog/how-to-optimize-momentjs-with-webpack
     // You can remove this if you don't use Moment.js:
     new webpack.IgnorePlugin(/^\.\/locale$/, /moment$/),
+
+    // Warn about duplicate dependencies.
+    // See https://github.com/facebookincubator/create-react-app/issues/1844
+    new DuplicatePackageCheckerPlugin({ strict: false }),
   ],
   // Some libraries import Node modules but don't use them in the browser.
   // Tell Webpack to provide empty mocks for them so importing them works.

packages/react-scripts/config/webpack.config.prod.js

@@ -18,6 +18,7 @@ const InterpolateHtmlPlugin = require('react-dev-utils/InterpolateHtmlPlugin');
 const SWPrecacheWebpackPlugin = require('sw-precache-webpack-plugin');
 const eslintFormatter = require('react-dev-utils/eslintFormatter');
 const ModuleScopePlugin = require('react-dev-utils/ModuleScopePlugin');
+const DuplicatePackageCheckerPlugin = require('duplicate-package-checker-webpack-plugin');
 const paths = require('./paths');
 const getClientEnvironment = require('./env');
 
@@ -356,6 +357,10 @@ module.exports = {
     // https://github.com/jmblog/how-to-optimize-momentjs-with-webpack
     // You can remove this if you don't use Moment.js:
     new webpack.IgnorePlugin(/^\.\/locale$/, /moment$/),
+
+    // Warn about duplicate dependencies.
+    // See https://github.com/facebookincubator/create-react-app/issues/1844
+    new DuplicatePackageCheckerPlugin({ strict: false }),
   ],
   // Some libraries import Node modules but don't use them in the browser.
   // Tell Webpack to provide empty mocks for them so importing them works.

packages/react-scripts/fixtures/kitchensink/.template.dependencies.json

@@ -7,7 +7,7 @@
     "jsdom": "9.8.3",
     "mocha": "3.2.0",
     "normalize.css": "7.0.0",
-    "prop-types": "15.5.6",
+    "prop-types": "^15.6.0",
     "test-integrity": "1.0.0"
   }
 }

packages/react-scripts/package.json

@@ -33,6 +33,7 @@
     "css-loader": "0.28.7",
     "dotenv": "4.0.0",
     "dotenv-expand": "4.0.1",
+    "duplicate-package-checker-webpack-plugin": "^2.1.0",
     "eslint": "4.10.0",
     "eslint-config-react-app": "^2.0.1",
     "eslint-loader": "1.9.0",