Generated by SassMeister.com.

SassMeister-input.scss

// ----
// Sass (v3.3.10)
// Compass (v1.0.0.alpha.20)
// ----

$register: ();

@mixin register ($key) {
  @if not index($register, $key) {
    $register: append($register, $key);
  }
}

@include register(foo);
@include register(bar);
@include register(bar);
@include register(baz);

body {
  content: $register;
}

SassMeister-output.css

body {
  content: foo bar baz;
}