I place this function atop my main/primary SASS file for all modules with in a site or project. When multiple modules are combined in a layout the main/primary styles are not duplicated each time. Thanks to user courtsimas and !default http://sass-lang.com/docs/yardoc/file.SASS_REFERENCE.html#variable_defaults_

gistfile1.txt

$imported-once-files: () !default;

@function import-once($name) {
  @if index($imported-once-files, $name) {
    @return false;
  }
  $imported-once-files: append($imported-once-files, $name);
  @return true;
}

@if import-once("_style.scss"){
  //base styles and declarations of cool sass-iness that you only want included once
}