emberjs/data — pull request #1504 by erundook (2013-11-16 13:59:08+00:00)
Title: Reject records that have IDs which are already in store

When app has a push-data (i.e. websocket) connection, this could happen: push update comes before the actual save() call is done, which saves another copy of the record when it finally gets an ID from the backend.