title: Sass Placeholders from Mixins
body: <p>I want to create Sass placeholders on the fly using arbitrary values passed from a style block:</p>
  
  <pre><code>@mixin example-mixin($arg) {
      %placeholder-#{$arg} {
          property: $arg;
      }
      @extend %placeholder-#{$arg};
  }
  </code></pre>
  
  <p>Calling the mixin:</p>
  
  <pre><code>.classname {
      @include example-mixin('value');
  }
  </code></pre>
  
  <p>This almost works, but for some reason in the CSS output the <code>.classname</code> is given twice as though it's a descendant selector:</p>
  
  <pre><code>.classname .classname {
      property: value;
  }
  </code></pre>
  
  <p>I'm not seeing the logic behind the duplicate class names - can anyone see what I'm doing wrong and/or suggest a workaround?</p>