Call #uniq on the list of files to ensure files are not uploaded twice.

When I run `assets:precompile` for my project the manifest contains lines like:

```yml
application.js: application-abcdefghijklmnopqrstuvwxyz123456.js
application/index.js: application-abcdefghijklmnopqrstuvwxyz123456.js
```

When using asset_sync with the `manifest` option enabled, this results in duplicate entries in the `local_files` array.  This patch simply calls `Array#uniq` after finding local files to remove these duplicates.