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 static void assertSnapshotStatusCountOnRepo(String otherBlockedRepoName, int count) {
        final SnapshotsStatusResponse snapshotsStatusResponse = client().admin()
            .cluster()
            .prepareSnapshotStatus(otherBlockedRepoName)
            .get();
        final List<SnapshotStatus> snapshotStatuses = snapshotsStatusResponse.getSnapshots();
        assertThat(snapshotStatuses, hasSize(count));
    }
    private ActionFuture<AcknowledgedResponse> startDeleteFromNonMasterClient(String repoName, String snapshotName) {
        logger.info("--> deleting snapshot [{}] from repo [{}] from non master client", snapshotName, repoName);
        return internalCluster().nonMasterClient().admin().cluster().prepareDeleteSnapshot(repoName, snapshotName).execute();
    }
    private ActionFuture<CreateSnapshotResponse> startFullSnapshotFromNonMasterClient(String repoName, String snapshotName) {
        logger.info("--> creating full snapshot [{}] to repo [{}] from non master client", snapshotName, repoName);
        return internalCluster().nonMasterClient()
            .admin()
            .cluster()
            .prepareCreateSnapshot(repoName, snapshotName)
            .setWaitForCompletion(true)
            .execute();
    }
    private ActionFuture<CreateSnapshotResponse> startFullSnapshotFromDataNode(String repoName, String snapshotName) {
        logger.info("--> creating full snapshot [{}] to repo [{}] from data node client", snapshotName, repoName);
        return internalCluster().dataNodeClient()
            .admin()
            .cluster()
            .prepareCreateSnapshot(repoName, snapshotName)
            .setWaitForCompletion(true)
            .execute();