<===> input.scss
%x {
  color: red;
}

foo {
  width: 10px;
  @extend %x;
}

hux {
  height: 12px;
  @extend %x;
}
<===> output.css
hux, foo {
  color: red;
}

foo {
  width: 10px;
}

hux {
  height: 12px;
}