Simple webpack config

webpack.conf.js

// webpack.config.js
var webpack = require("webpack");
var ExtractTextPlugin = require("extract-text-webpack-plugin");
var path = require("path");

module.exports = {
  // The standard entry point and output config
  entry: {
    app: "./app"
  },
  output: {
    filename: "[name].js",
    path: path.join(__dirname, "./css"),
    publicPath: "/",
  },
  resolve: {
    extensions: [
      "",
      ".js",
      ".css"
    ],
  },
  cssnext: {
    url: false,
    browsers : ['ff >= 20', 'ie >= 8', 'safari >= 5.1', 'opera >= 12', 'chrome >=20'],
    import: {
      path: ["./front.css"]
    }
  },
  module: {
    loaders: [
    // Extract css files
      {
       test: /\.css$/,
        loader: ExtractTextPlugin.extract(
          "style-loader", "css-loader!cssnext-loader"
        )
      },
      { test: /\.png$/, loader: "file-loader" }
    ]
  },
  // Use the plugin to specify the resulting filename (and add needed behavior to the compiler)
  plugins: [
    new ExtractTextPlugin("front.css")
  ]
}

Do you use @import in your cssnext files? If so you are ending up with duplicate code in the final bundle most definitely.