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/main/java/org/elasticsearch/repositories/RepositoriesService.java
}
    private static RepositoryConflictException newRepositoryConflictException(String repository, String reason) {
        return new RepositoryConflictException(
            repository,
            "trying to modify or unregister repository that is currently used (" + reason + ')',
            "trying to modify or unregister repository [" + repository + "] that is currently used (" + reason + ')'
        );
    }
    public List<BiConsumer<Snapshot, Version>> getPreRestoreVersionChecks() {
        return preRestoreChecks;
    }
    @Override
    protected void doStart() {
    }
    @Override
    protected void doStop() {
    }
    @Override
    protected void doClose() throws IOException {
        clusterService.removeApplier(this);
        final Collection<Repository> repos = new ArrayList<>();
        repos.addAll(internalRepositories.values());
        repos.addAll(repositories.values());
        IOUtils.close(repos);
        for (Repository repo : repos) {
            repo.awaitIdle();
        }
    }
}