Duplicate entries in suggestion list

[Peruibeloko]

This is not the same issue as #9

Autocomplete suggestions are duplicated for variables used twice or more on a file.

How to reproduce

1. Create two CSS files
2. On the first file, declare a variable with any name
3. On the second file, use said variable
4. Still on the second file, type -- for listing the available variables

On the displayed list, you'll have two entries for the variable:

• One which comes from VS Code itself. Since the variable was previously used in the file, the native CSS support recognizes it and adds it as a suggestion.
• Another one which comes from the extension. It pulls the variable name directly from its definition in the first file.

Evidence

For the record: I'm not using SCSS or Sass, nesting is now available as a native feature of CSS itself