Fixed a bug that in some cases SASS files could be included multiple times

A bug in the theme tool chain caused that the SCSS files of a brick were included multiple times in some cases. As the files were loaded in a way that these duplicates might also have been overriding other styling rules and this will no longer happen after fixing the bug this is considered a potentionally breaking change. For that reason the fix is for now only enabled if the feature flag "enableDuplicateSassImportsFix" is set in the theme.config.json like this:

{
  ...
  "enableDuplicateSassImportsFix": true
}

(CMS-19627)