minimal test for self-extending mixin pattern

SassMeister-input.scss

// ----
// Sass (v3.4.14)
// Compass (v1.0.3)
// sass-maps-plus (ve4d32a1dc2)
// ----

@import "sass-maps-plus";

$mixins: ();

@mixin margin($arg: 0) {

  $id: get($mixins, 'margin', $arg);

  @if not $id {
    $id: unique-id();
    $mixins: merge($mixins, 'margin', $arg, $id) !global;
    @at-root %#{$id} {
      margin: $arg + em;
    }
  }
  @extend %#{$id};
}

// TESTS

.test {
  @include margin(2);
}

.test2 {
  @include margin(2);
}

.test3 {
  @include margin(2);
}

SassMeister-output.css

.test, .test2, .test3 {
  margin: 2em;
}