CSS, SCSS, LESS support for Sublime's LSP plugin provided through VS Code's CSS language server.
- Install LSP and 
LSP-cssfrom Package Control. If you use Sass install Sass. If you use Less install LESS. - Restart Sublime.
 
There are some ways to configure the package and the language server.
- From 
Preferences > Package Settings > LSP > Servers > LSP-css - From the command palette 
Preferences: LSP-css Settings 
- 
I'm getting duplicate suggestions in the completion popup.
Sublime Text comes with a plugin that provides completions for the CSS and HTML syntaxes. This results in duplicate completions when using this package.
To disable ST's built-in completions for CSS syntax, for example:
- open any CSS file
 - run 
Preferences: Settings - Syntax Specificfrom theCommand Pallete - add 
"disable_default_completions": truein the view on the right 
Repeat for other syntaxes that provide built-in completions. Same applies to third-party SASS package that provides SASS and SCSS support.