Ensure aliases aren't double added

Author: philipwalton

lib/account-summaries/account-summaries.js

@@ -245,7 +245,7 @@ alias(AccountSummaries.prototype, 'getAccountByWebPropertyId',
  * @param {string} aliasName The reference property's alias name.
  */
 function alias(object, referenceProp, aliasName) {
-  if (Object.defineProperty) {
+  if (Object.defineProperty && !Object.hasOwnProperty.call(object, aliasName)) {
     Object.defineProperty(object, aliasName, {
       get: function() {
         return object[referenceProp];