emberjs/ember.js — pull request #1311 by elucid (2012-08-23 04:15:27+00:00)
Title: Don't add duplicates in SortableMixin#addObject

The docs for MutableEnumerable state that addObject should "add the passed object to the receiver if the object is not already present in the collection" but the current implementation in SortableMixin adds objects unconditionally. As a result, this also breaks the behavior for ArrayControllers.