Update lib/rules/prop-name-casing.js

Co-authored-by: Flo Edelmann <git@flo-edelmann.de>

Author: westberliner

lib/rules/prop-name-casing.js

@@ -69,8 +69,13 @@ module.exports = {
       {
         type: 'object',
         properties: {
-          ignores: { type: 'array' }
-        }
+          ignores: {
+            type: 'array',
+            items: { type: 'string' },
+            uniqueItems: true
+          }
+        },
+        additionalProperties: false
       }
     ],
     messages: {