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
+ "because it uses snap-*.data files and not the index-N to determine what files to restore"
        );
        client().admin().indices().prepareDelete("test-idx-1", "test-idx-2").get();
        RestoreSnapshotResponse restoreSnapshotResponse = clusterAdmin().prepareRestoreSnapshot("test-repo", "test-snap-1")
            .setWaitForCompletion(true)
            .get();
        assertEquals(0, restoreSnapshotResponse.getRestoreInfo().failedShards());
    }
    private void assertRepositoryBlocked(Client client, String repo, String existingSnapshot) {
        logger.info("--> try to delete snapshot");
        final RepositoryException repositoryException3 = expectThrows(
            RepositoryException.class,
            () -> client.admin().cluster().prepareDeleteSnapshot(repo, existingSnapshot).execute().actionGet()
        );
        assertThat(
            repositoryException3.getMessage(),
            containsString("Could not read repository data because the contents of the repository do not match its expected state.")
        );
        logger.info("--> try to create snapshot");
        final RepositoryException repositoryException4 = expectThrows(
            RepositoryException.class,
            () -> client.admin().cluster().prepareCreateSnapshot(repo, existingSnapshot).execute().actionGet()
        );
        assertThat(
            repositoryException4.getMessage(),
            containsString("Could not read repository data because the contents of the repository do not match its expected state.")
        );
    }
}