Multiple @extend rules in the same block don't process the second @extend correctly

```css
%foo {
right: 0;
}

%bar {
left: 0;
}

.baz {
@extend %foo;
@extend %bar;
}
```

Worked in 2.0.0, but doesn't generate correctly on 3.0.0.

Start by reproducing the CSS example with the current implementation and compare its generated output with version 2.0.0 and 3.0.0. Done means both @extend rules in the same block are processed and the generated CSS reflects both extensions correctly.

Written by the indexing model from the issue text.