<===> input.scss
$color: "black";
$color: red;
$background: "blue";

a {
  color: $color;
  background: $background;
}

$y: before;

$x: 1 2 $y;

foo {
  a: $x;
}

$y: after;

foo {
  a: $x;
}
<===> output.css
a {
  color: red;
  background: "blue";
}

foo {
  a: 1 2 before;
}

foo {
  a: 1 2 before;
}