webpack + postcss + extract-text-plugin = no CSS duplicates

webpack.config.js

postcss: () => {
  return [
    cssnano({
      reduceIdents: false,
      discardDuplicates: true,
      autoprefixer: true
    })
  ];
},modules: {
  loaders: [{
    test: test_expressions.css,
    loader: extractTextPlugin.extract('css-loader?importLoaders=1!postcss-loader')
  },{
    test: test_expressions.stylus,
    loader: extractTextPlugin.extract('css-loader?importLoaders=2!postcss-loader!stylus-loader')
  }]
},
plugins: [
  new extractTextPlugin('[name].css'),
  new OptimizeCssAssetsPlugin({
    assetNameRegExp: /\.css$/
  })
]

If you want to use this in dev, you will have to use a plugin like write-file-webpack-plugin to force webpack to write the file to disk in dev.