NSFetchedResultsController returns duplicates after merging contexts

I notice weird issues where NSFetchedResultsController returns in its fetchedObjects two instances of the same object.

func controllerDidChangeContent(_ controller: NSFetchedResultsController<NSFetchRequestResult>) {
     items = (controller.fetchedObjects ?? []) as! [Item]
        
     print(items)
       // Prints:
       // - 0 : <Item: 0x600000d40e10> (entity: Item; id: 0x600002e6b2e0 <x-coredata:///Item/tE1646DB0-C3C2-4AE1-BC32-6B10934F292C2>; ....
       // - 1 : <Item: 0x600000d40e10> (entity: Item; id: 0x600002e6b2e0 <x-coredata:///Item/tE1646DB0-C3C2-4AE1-BC32-6B10934F292C2>;  ...
        
     print(items[0] == items[1]) // true !!!
}

The issue occurs after adding a new item and saving both contexts.

func weirdTest() {
   print("1. Adding.")
   let item = Item(context: viewContext)
   item.id = UUID()
   viewContext.processPendingChanges()
     
   print("2. Saving.")
   viewContext.performAndWait {
      try! viewContext.save()
   }
   rootCtx.performAndWait {
      try! rootCtx.save()
   }
}

Here's my Core Data Stack:

View Context (main thread) --> Background Context --> Persistent Store Coordinator

Ahd here's how I configure my contexts:

lazy var rootCtx: NSManagedObjectContext = {
     let rootCtx = NSManagedObjectContext(concurrencyType: .privateQueueConcurrencyType)
     rootCtx.persistentStoreCoordinator = coordinator
     rootCtx.automaticallyMergesChangesFromParent = true
     return rootCtx
}()
    

lazy var viewContext: NSManagedObjectContext = {
     let ctx = NSManagedObjectContext(concurrencyType: .mainQueueConcurrencyType)
     ctx.parent = rootCtx
     ctx.automaticallyMergesChangesFromParent = true
     return ctx
 }()

Anyone has any idea what's going on here? :<