NSFetchedResultsController returns duplicates (the same objectID and reference)

I'm experiencing a weird behaviour with NSFetchedResultsController. It is new in iOS 10. Straight to the point: `fetchedObjects` contains duplicate objects.

This is not by any mean "duplicate" in my own criteria. They literally have the same objectIDs and the same reference.

How does it happen in my app:

1. FRC is setup and fetch performed
2. 2 objects are fetched (in this example)
3. Something happens in my sync layer, another object (that fetch items are in relationship with) is updated
4. FRC calls delegate that the content did change. `fetchedObjects` contains 4 objects (2 duplicates)

Here's a console output from my debugging in `controllerDidChangeContent` method.

    po frc.fetchedObjects
  
        - 0 : <ListItem: 0x1700dc3f0> (entity: ListItem; id: 0x1706294a0 <x-coredata://12D0CB00-7BF4-402A-8371-19DD1CFB1537/ListItem/t3384FC2D-3399-41FE-B7DD-C277922F495445> ; data: { ... }
  
        - 1 : <ListItem: 0x1700dc3f0> (entity: ListItem; id: 0x1706294a0 <x-coredata://12D0CB00-7BF4-402A-8371-19DD1CFB1537/ListItem/t3384FC2D-3399-41FE-B7DD-C277922F495445> ; data: { ... }
  
        - 2 : <ListItem: 0x1704c49f0> (entity: ListItem; id: 0x170631680 <x-coredata://12D0CB00-7BF4-402A-8371-19DD1CFB1537/ListItem/t3384FC2D-3399-41FE-B7DD-C277922F495446> ; data: { ... }
  
        - 3 : <ListItem: 0x1704c49f0> (entity: ListItem; id: 0x170631680 <x-coredata://12D0CB00-7BF4-402A-8371-19DD1CFB1537/ListItem/t3384FC2D-3399-41FE-B7DD-C277922F495446> ; data: { ... }
  
  
    (lldb) po frc.fetchedObjects![0].objectID.isEqual(frc.fetchedObjects![1].objectID)
    true

Another note: when I recreate FRC and fetch objects again, there are no duplicates.

I simply have no idea what could be causing this problem. I might be doing something wrong in my app that is causing that bug. It's really weird anyway that FRC allows for duplicates objects in `fetchedObjects`.

Can you please give me any clue of how should I debug it?