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
final String dataNode2 = internalCluster().startDataOnlyNode();
        final String index1 = "index-1";
        final String index2 = "index-2";
        createIndexWithContent(index1, dataNode1, dataNode2);
        createIndexWithContent(index2, dataNode2, dataNode1);
        final String repository = "test-repo";
        createRepository(repository, "mock");
        createFullSnapshot(repository, "snapshot-1");
        index(index1, "some_doc", Map.of("foo", "bar"));
        index(index2, "some_doc", Map.of("foo", "bar"));
        blockAndFailDataNode(repository, dataNode1);
        blockDataNode(repository, dataNode2);
        final ActionFuture<CreateSnapshotResponse> snapshotBlocked = startFullSnapshot(repository, "snapshot-2");
        waitForBlock(dataNode1, repository);
        waitForBlock(dataNode2, repository);
        unblockNode(repository, dataNode1);
        assertAcked(clusterAdmin().prepareCloneSnapshot(repository, "snapshot-1", "target-1").setIndices(index1).get());
        unblockNode(repository, dataNode2);
        snapshotBlocked.get();
        assertThat(
            clusterAdmin().prepareSnapshotStatus().setSnapshots("target-1").setRepository(repository).get().getSnapshots(),
            hasSize(1)
        );
        createFullSnapshot(repository, "snapshot-3");
    }
    public void testIndexDeletedWhileSnapshotQueuedAfterClone() throws Exception {
        final String master = internalCluster().startMasterOnlyNode(LARGE_SNAPSHOT_POOL_SETTINGS);
        internalCluster().startDataOnlyNode();
        final String index1 = "index-1";
        final String index2 = "index-2";