How to sync icloud coredata?

gistfile1.txt

2 Devices A and B, they both receive a push notification at the same time, 
which triggers a fetch of an object on a remote server, and store the content 
inside a coredata icloud enabled DB. The object has a PID stored on the remote
server.

You end up with 2 entries: one fetched on the remote server, one synced 
from the other device through coredata sync.

Both devices have different NSManagedID for the same entries (you can't 
automatically remove the most recent one based one it), the only thing you 
can be doing is randomly removing duplicates after icloud sync, but that's not very 
efficient. You may also remove the latest entry the user read.

I tried added an automatic updated_at for each update, but that doesn't 
work either. My last idea is storing on the server a flag the item has already been
downloaded, and not store it on the 2nd device if it did.