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
unblockNode(repository, master);
        assertAcked(cloneSnapshot.get());
        assertAcked(startDeleteSnapshot(repository, cloneTarget).get());
        assertThat(
            clusterAdmin().prepareSnapshotStatus().setSnapshots("snapshot-2", "snapshot-3").setRepository(repository).get().getSnapshots(),
            hasSize(2)
        );
    }
    public void testIndexDeletedWhileSnapshotAndCloneQueuedAfterClone() throws Exception {
        final String master = internalCluster().startMasterOnlyNode(LARGE_SNAPSHOT_POOL_SETTINGS);
        internalCluster().startDataOnlyNode();
        final String index1 = "index-1";
        final String index2 = "index-2";
        createIndexWithContent(index1);
        createIndexWithContent(index2);
        final String repository = "test-repo";
        createRepository(repository, "mock");
        final String sourceSnapshot = "source-snapshot";
        createFullSnapshot(repository, sourceSnapshot);
        final IndexId index1Id = getRepositoryData(repository).resolveIndexId(index1);
        blockMasterOnShardLevelSnapshotFile(repository, index1Id.getId());
        final String cloneTarget = "target-snapshot";
        final ActionFuture<AcknowledgedResponse> cloneSnapshot = clusterAdmin().prepareCloneSnapshot(
            repository,
            sourceSnapshot,
            cloneTarget
        ).setIndices(index1, index2).execute();
        awaitNumberOfSnapshotsInProgress(1);
        waitForBlock(master, repository);
        final ActionFuture<CreateSnapshotResponse> snapshot3 = clusterAdmin().prepareCreateSnapshot(repository, "snapshot-3")