minifier: remove duplicate selectors

[yisibl]

h1,h2,h3,h1,h3{
  color:blue
}

div.class,div.class,.class{
  color:blue
}

https://parcel-css.vercel.app/#%7B%22minify%22%3Atrue%2C%22nesting%22%3Atrue%2C%22customMedia%22%3Atrue%2C%22cssModules%22%3Afalse%2C%22analyzeDependencies%22%3Afalse%2C%22targets%22%3A%7B%22chrome%22%3A6225920%7D%2C%22source%22%3A%22h1%2Ch2%2Ch3%2Ch1%2Ch3%7B%5Cn%20%20color%3Ablue%5Cn%7D%5Cn%5Cndiv.class%2Cdiv.class%2C.class%7B%5Cn%20%20color%3Ablue%5Cn%7D%22%2C%22unusedSymbols%22%3A%5B%5D%2C%22version%22%3A%22local%22%7D

[devongovett]

Seems reasonable. When does this happen in real-world code though?

[yisibl]

Seems reasonable. When does this happen in real-world code though?

I'm not quite sure if there might be such a code, my guess is that it might be a clerical error or generated by the preprocessor.

[LeoniePhiline]

I'd like to take a look at this one.

[LeoniePhiline]

I won't manage currently to work on this. Free to take!

[R-Bower]

For anybody still looking for a solution here, I've managed to combine cssnano and postcss-lightningcss to achieve the desired result. Note that this will require postcss.

[qsliu2017]

I'd like to have a try