Seen in soak testing:

```
org.elasticsearch.snapshots.SnapshotStressTestsIT > testRandomActivities {seed=[AB70F7282B153829:AB3327F808885EA1]} FAILED
    java.lang.AssertionError: add restore for [[index-0][2]] that already has an existing restore
        at __randomizedtesting.SeedInfo.seed([AB70F7282B153829]:0)
        at org.elasticsearch.repositories.blobstore.BlobStoreRepository.restoreShard(BlobStoreRepository.java:3399)
        at org.elasticsearch.index.shard.StoreRecovery.lambda$restore$14(StoreRecovery.java:569)
        at org.elasticsearch.action.ActionListenerImplementations$ResponseWrappingActionListener.onResponse(ActionListenerImplementations.java:236)
        at org.elasticsearch.action.support.SubscribableListener$SuccessResult.complete(SubscribableListener.java:310)
        at org.elasticsearch.action.support.SubscribableListener.tryComplete(SubscribableListener.java:230)
        at org.elasticsearch.action.support.SubscribableListener.addListener(SubscribableListener.java:133)
        at org.elasticsearch.action.support.SubscribableListener.addListener(SubscribableListener.java:108)
        at org.elasticsearch.index.shard.StoreRecovery.lambda$restore$15(StoreRecovery.java:567)
        at org.elasticsearch.action.ActionListenerImplementations$DelegatingFailureActionListener.onResponse(ActionListenerImplementations.java:212)
        at org.elasticsearch.action.ActionListenerImplementations$DelegatingResponseActionListener.onResponse(ActionListenerImplementations.java:182)
        at org.elasticsearch.index.CompositeIndexEventListener.callListeners(CompositeIndexEventListener.java:278)
        at org.elasticsearch.index.CompositeIndexEventListener.iterateBeforeIndexShardRecovery(CompositeIndexEventListener.java:287)
        at org.elasticsearch.index.CompositeIndexEventListener.beforeIndexShardRecovery(CompositeIndexEventListener.java:314)
        at org.elasticsearch.index.shard.IndexShard.preRecovery(IndexShard.java:1683)
        at org.elasticsearch.index.shard.StoreRecovery.restore(StoreRecovery.java:519)
        at org.elasticsearch.index.shard.StoreRecovery.recoverFromRepository(StoreRecovery.java:311)
        at org.elasticsearch.index.shard.IndexShard.restoreFromRepository(IndexShard.java:2349)
        at org.elasticsearch.index.shard.IndexShard.lambda$startRecovery$21(IndexShard.java:3132)
        at org.elasticsearch.action.ActionRunnable$3.doRun(ActionRunnable.java:73)
        at org.elasticsearch.common.util.concurrent.ThreadContext$ContextPreservingAbstractRunnable.doRun(ThreadContext.java:983)
        at org.elasticsearch.common.util.concurrent.AbstractRunnable.run(AbstractRunnable.java:26)
        at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1144)
        at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:642)
        at java.base/java.lang.Thread.run(Thread.java:1583)
```

~This looks to happen if we attempt to do two concurrent restores of an index, one where the index is closed and the other where it is deleted. This test shouldn't do that since it is focussing on the happy path, and one of those restores should fail, but nor should it be possible to trip an assertion if we try it.~

Edit: hmm no that doesn't make sense, there's some confusing logging in this area. Looking again I see that we restore the same index twice in quick succession, and there are some recovery cancellations in the logs. Cancelling a snapshot-based recovery won't stop the restore (and clean up the `ongoingRestores` marker) straight away. But I'm not sure how that could happen in `SnapshotStressTestsIT` yet, it looks like we always hold all the permits for an index until the restore succeeds. 

---

I think the production consequences of this are mild, we mostly track the ongoing restores so that tests can wait for the repository to become idle.
server/src/main/java/org/elasticsearch/repositories/blobstore/BlobStoreRepository.java
// 
                final Set<String> blobNamesToIgnore = snapshotIds.stream()
                    .flatMap(
                        snapshotId -> Stream.of(
                            GLOBAL_METADATA_FORMAT.blobName(snapshotId.getUUID()),
                            SNAPSHOT_FORMAT.blobName(snapshotId.getUUID()),
                            INDEX_FILE_PREFIX + newestStaleRepositoryDataGeneration
                        )
                    )
                    .collect(Collectors.toSet());
                final List<String> blobsToLog = blobsToDelete.stream().filter(b -> blobNamesToIgnore.contains(b) == false).toList();
                if (blobsToLog.isEmpty() == false) {
                    logger.info("[{}] Found stale root level blobs {}. Cleaning them up", metadata.name(), blobsToLog);
                }
            }
        }
    }
    @Override
    public void finalizeSnapshot(final FinalizeSnapshotContext finalizeSnapshotContext) {
        final long repositoryStateId = finalizeSnapshotContext.repositoryStateId();
        final ShardGenerations shardGenerations = finalizeSnapshotContext.updatedShardGenerations();
        final SnapshotInfo snapshotInfo = finalizeSnapshotContext.snapshotInfo();
        assert repositoryStateId > RepositoryData.UNKNOWN_REPO_GEN
            : "Must finalize based on a valid repository generation but received [" + repositoryStateId + "]";
        final Collection<IndexId> indices = shardGenerations.indices();
        final SnapshotId snapshotId = snapshotInfo.snapshotId();
        // 
        // 
        // 
        //