Add new error "duplicate request" (#1189)

Author: mtrezza

src/ParseError.js

@@ -353,6 +353,15 @@ ParseError.FILE_DELETE_ERROR = 153;
  */
 ParseError.REQUEST_LIMIT_EXCEEDED = 155;
 
+/**
+ * Error code indicating that the request was a duplicate and has been discarded due to
+ * idempotency rules.
+ * @property DUPLICATE_REQUEST
+ * @static
+ * @final
+ */
+ParseError.DUPLICATE_REQUEST = 159;
+
 /**
  * Error code indicating an invalid event name.
  * @property INVALID_EVENT_NAME