Import Once Feature Implemented in Sass.

include-once.scss

$is-included : () !default;

@mixin once( $name ) {
    @if include-once( $name ) {
        @content;
    }
}

@function include-once( $name ) {
    @if index( $is-included, $name ) {
        @return false;
    }

    $is-included : append( $is-included, $name );

    @return true;
}

Thanks to sass/sass#156 and https://gist.github.com/courtsimas/1167053 for the initial idea.