Double imports occur

If you have multiple mentions of an import, the file is included multiple times.

This is easy to fix in \Leafo\ScssPhp\Compiler

```
protected function importFile($path, $out)
{
    // see if tree is cached
    $realPath = realpath($path);
    if (isset($this->importCache[$realPath])) {
        // Nothing to do, already imported.
        return;
```
