Proposed fix when @extending the same thing more than once

Applied @nessy's patch from issue 29 but there are new errors reported by phpunit.

For example:
```
.hoverlink { @extend a:hover }
a:hover { text-decoration: underline }
#demo .overview .fakelink {@extend a}
```
scssphp (before) -- wrong in retrospect:
```
a:hover, .hoverlink {
   text-decoration: underline; }
```
scssphp (after):
```
a:hover, .hoverlink:hover, #demo .overview .fakelink:hover {
   text-decoration: underline; }
```
sass-lang.com:
```
a:hover, .hoverlink, #demo .overview .fakelink:hover {
  text-decoration: underline; }
```