"merge components into shorthand properties" (level 2) causes invalid repetition of !import

[pocketjoso]

Hi again, continuous thanks for providing this excellent library! Long time since I last found a bug, but here's a new one, hoping it will be easy to fix. :)

Environment

• clean-css version - npm ls clean-css:
  clean-css@4.1.11. Also reproduced via online demo (https://jakubpawlowicz.github.io/clean-css/)
• node.js version - node -v:
  10.0.0 (also online demo)
• operating system:
  MacOS (also online demo)

Configuration options

I run with level: 2, but I broke it down more specifically to the merge components into shorthand properties being responsible, via the online demo.

Input CSS

div {
    border-color: currentColor!important;
    border-radius: 0!important;
    border-style: none!important;
    border-width: medium!important;
    list-style-type: inherit!important;
    list-style-position: outside!important;
    list-style-image: none!important;
}

Actual output CSS

div{border-radius:0!important;border:currentColor!important!important!important;border:currentColor!important!important!important;border:currentColor!important!important!important;list-style:outside;list-style-type:inherit!important}

This broken rule repeats badly, taking up more and more space the larger the file. It made the original css file about 3 times larger than before clean-css (before I pruned it for debugging and presenting here).

Expected output CSS

Haven't reviewed this with 100% certainty; the main thing is just to not have the incorrect !important!important.. repetition. But it also seems that there is a missing important on the list-style property.

div{border-radius:0!important;border:currentColor;list-style:outside!important;list-style-type:inherit!important}

[pocketjoso]

Hi, I don't want to be annoying, just want to ask if you have had a chance to look at this at all? Thanks!