The donor shard of the movePrimary invokes the _shardsvrCloneCatalogData command to ask the recipient to clone the unsharded collections of the given database.

If for any reason the recipient fails to clone the catalog data (e.g., the node steps down), the donor automatically retries the command, but it may fail with the NamespaceExists error. This occurs because the previous call may have cloned the data, partially or entirely.

Since _shardsvrCloneCatalogData is not idempotent, and the entire cloning phase of the movePrimary would be repeated after cleaning up any orphaned data on the recipient, the command must be invoked once.