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/ConcurrentSnapshotsIT.java
}
    private ActionFuture<CreateSnapshotResponse> startFullSnapshotFromMasterClient(String repoName, String snapshotName) {
        logger.info("--> creating full snapshot [{}] to repo [{}] from master client", snapshotName, repoName);
        return internalCluster().masterClient()
            .admin()
            .cluster()
            .prepareCreateSnapshot(repoName, snapshotName)
            .setWaitForCompletion(true)
            .execute();
    }
    private void createIndexWithContent(String indexName, String nodeInclude, String nodeExclude) {
        createIndexWithContent(
            indexName,
            indexSettingsNoReplicas(1).put("index.routing.allocation.include._name", nodeInclude)
                .put("index.routing.allocation.exclude._name", nodeExclude)
                .build()
        );
    }
    private static boolean snapshotHasCompletedShard(String repoName, String snapshot, SnapshotsInProgress snapshotsInProgress) {
        for (SnapshotsInProgress.Entry entry : snapshotsInProgress.forRepo(repoName)) {
            if (entry.snapshot().getSnapshotId().getName().equals(snapshot)) {
                for (SnapshotsInProgress.ShardSnapshotStatus shard : entry.shards().values()) {
                    if (shard.state().completed()) {
                        return true;
                    }
                }
            }
        }
        return false;
    }