Incorporating review comment

Author: yanapu-jyothi

file-formats/msag/msag-v1.json

@@ -61,6 +61,7 @@
       "title": "Messages",
       "description": "Messages",
       "type": "array",
+      "uniqueItems": true,
       "items": {
         "title": "Message Properties",
         "description": "Message properties",