postcss-unique-selectors 
Ensure CSS selectors are unique.
Install
With npm do:
npm install postcss-unique-selectors --save
Example
Selectors are sorted naturally, and deduplicated:
Input
h1,h3,h2,h1 {
color: red
}
Output
h1,h2,h3 {
color: red
}
Usage
See the PostCSS documentation for examples for your environment.
Contributing
Pull requests are welcome. If you add functionality, then please add unit tests to cover it.
License
MIT © Ben Briggs