Update .stylelintrc.yml

Author: philwareham

.stylelintrc.yml

@@ -146,6 +146,9 @@ rules:
   # Require a single space or disallow whitespace before the bang of declarations.
   declaration-bang-space-before: always
 
+  # Disallow duplicate custom properties within declaration blocks.
+  declaration-block-no-duplicate-custom-properties: true
+
   # Disallow duplicate properties within declaration blocks.
   #declaration-block-no-duplicate-properties: