In some examples, in response to an event at the deduplication system, a system accesses item metadata of a backup item that is backed up to a remote object storage system, the item metadata of the backup item including range information indicating a range of identifier values for portion objects of the backup item stored in the remote object storage system. The system issues, based on the range information, requests to obtain respective attribute information of the portion objects of the backup item stored in the remote object storage system. The system determines, based on the attribute information, a name of a given portion object of the backup item already used.