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
createFullSnapshot(repository, sourceSnapshot);
        indexDoc(index2, "doc_id", "foo", "bar");
        blockNodeWithIndex(repository, index2);
        final String sn1 = "snapshot-1";
        final ActionFuture<CreateSnapshotResponse> snapshot1 = clusterAdmin().prepareCreateSnapshot(repository, sn1)
            .setIndices(index1, index2)
            .setWaitForCompletion(true)
            .execute();
        awaitNumberOfSnapshotsInProgress(1);
        final String targetSnapshot = "target-snapshot";
        final ActionFuture<AcknowledgedResponse> clone = clusterAdmin().prepareCloneSnapshot(repository, sourceSnapshot, targetSnapshot)
            .setIndices(index1)
            .execute();
        assertAcked(clone.get());
        unblockAllDataNodes(repository);
        assertSuccessful(snapshot1);
        logger.info("--> deleting snapshots [{},{}] from repo [{}]", sn1, sourceSnapshot, repository);
        assertAcked(clusterAdmin().prepareDeleteSnapshot(repository).setSnapshots(sn1, sourceSnapshot).get());
        assertThat(
            clusterAdmin().prepareSnapshotStatus().setSnapshots(targetSnapshot).setRepository(repository).get().getSnapshots(),
            hasSize(1)
        );
    }
    public void testOutOfOrderCloneFinalization() 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);