Today if we detect concurrent modification of a snapshot repository then we mark it with `CORRUPTED_REPO_GEN` which blocks further writes to try and prevent additional damage to its contents. You can remove the marker by calling the register-repository API again, but this API will discard no-op re-registrations without taking any action. You can work around this by unregistering the repository first (which may be impossible if it's still in use for searchable snapshots) or by re-registering it with some trivial setting change (which seems hacky).

We should not consider it a no-op re-registration if the repository is currently marked with `CORRUPTED_REPO_GEN`, because doing this re-registration will re-read the repo contents and clear the corruption marker and therefore is not a no-op.
server/src/internalClusterTest/java/org/elasticsearch/snapshots/CorruptedBlobStoreRepositoryIT.java
final String repoName = "test-repo";
        final Path repoPath = randomRepoPath();
        createRepository(repoName, "fs", repoPath);
        final String oldVersionSnapshot = initWithSnapshotVersion(repoName, repoPath, SnapshotsService.OLD_SNAPSHOT_FORMAT);
        final String indexName = "test-index";
        createIndex(indexName);
        createFullSnapshot(repoName, "snapshot-1");
        
        
        logger.info("--> move shard level metadata to new generation");
        final IndexId indexId = getRepositoryData(repoName).resolveIndexId(indexName);
        final Path shardPath = repoPath.resolve("indices").resolve(indexId.getId()).resolve("0");
        final Path initialShardMetaPath = shardPath.resolve(BlobStoreRepository.INDEX_FILE_PREFIX + "0");
        assertFileExists(initialShardMetaPath);
        Files.move(initialShardMetaPath, shardPath.resolve(BlobStoreRepository.INDEX_FILE_PREFIX + "1"));
        startDeleteSnapshot(repoName, oldVersionSnapshot).get();
        createFullSnapshot(repoName, "snapshot-2");
    }
    public void testRepairBrokenShardGenerations() throws Exception {
        final String repoName = "test-repo";
        final Path repoPath = randomRepoPath();
        createRepository(repoName, "fs", repoPath);
        final String oldVersionSnapshot = initWithSnapshotVersion(repoName, repoPath, SnapshotsService.OLD_SNAPSHOT_FORMAT);
        final String indexName = "test-index";
        createIndex(indexName);
        createFullSnapshot(repoName, "snapshot-1");
        startDeleteSnapshot(repoName, oldVersionSnapshot).get();
        logger.info("--> move shard level metadata to new generation and make RepositoryData point at an older generation");
        final IndexId indexId = getRepositoryData(repoName).resolveIndexId(indexName);
        final Path shardPath = repoPath.resolve("indices").resolve(indexId.getId()).resolve("0");