Return the correct object if the association already exists in add

lib/associations/has-many.js

@@ -375,7 +375,7 @@ module.exports = (function() {
             var Class = Object(association.through) === association.through ? HasManyDoubleLinked : HasManySingleLinked
             return new Class(association, instance).injectAdder(newInstance, additionalAttributes, !!currentAssociatedObjects.length)
           } else {
-            return Utils.Promise.resolve(currentAssociatedObjects[1])
+            return Utils.Promise.resolve(currentAssociatedObjects[0])
           }
         })
       }