Fix duplicated css imports

see webpack-contrib/sass-loader#972

webpack/base.js

@@ -31,6 +31,18 @@ function _resolveTheme(rootPath, indicoClientPath, filePath) {
   return path.resolve(rootPath, filePath);
 }
 
+function _importOnce(...args) {
+  // Due to https://github.com/webpack-contrib/sass-loader/pull/958/files we need to
+  // use a different context for the importOnce plugin in order to keep the cache
+  // between calls to this function.
+  // Using a global one might work as well but like this it's scoped with the same
+  // lifetime which we had before the sass-loader update as `options` is cloned at
+  // the same time the object previously used in `this` was created (`context` in
+  // `getOptions`).
+  this.options._ctx = this.options._ctx || {options: this.options};
+  return importOnce.apply(this.options._ctx, args);
+}
+
 export function getThemeEntryPoints(config, prefix) {
   const themes = config.themes;
   const indicoClientPath = path.join(
@@ -199,7 +211,7 @@ export function webpackDefaults(env, config, bundles, isPlugin = false) {
           sassOptions: {
             includePaths: [scssIncludePath],
             outputStyle: 'compact',
-            importer: [sassResolver, importOnce],
+            importer: [sassResolver, _importOnce],
           },
         },
       },