Remove duplicate stylesheets for RTL themes that use _directional.scss

mytheme_template.php

<?php

/**
 * Implements hook_css_alter().
 */
function mytheme_css_alter(&$css) {
  global $language;

  // If the current language is RTL, find and remove CSS files from this
  // theme that have the RTL overrides. Drupal's RTL methodology adds files
  // that it finds containing the '-rtl' suffix, but doesnt remove the
  // original LTR version. However, because _directional.scss generates
  // two separate stylesheets with the same rules the original (LTR) version
  // should be removed to avoid inflicting duplicate styles on RTL languages.
  if ($language->direction == LANGUAGE_RTL) {
    foreach ($css as $data => $item) {
      // Remove original (LTR) version of all RTL stylesheets from this
      // theme.
      if ($item['type'] == 'file'
        && strpos($item['data'], 'mytheme')
        && strpos($item['data'], '-rtl.css')) {
        $ltr = str_replace('-rtl.css', '.css', $item['data']);
        unset($css[$ltr]);
      }
    }
  }
}

Why would you need this? See blog the post where this snippet further explained:

http://jwilson3.postach.io/right-to-left-theming-in-drupal-with-directional-scss