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
assertSuccessful(snapshot2);
        assertThat(
            clusterAdmin().prepareSnapshotStatus().setSnapshots("snapshot-2").setRepository(repository).get().getSnapshots(),
            hasSize(1)
        );
    }
    public void testCloneQueuedAfterMissingShard() throws Exception {
        final String master = internalCluster().startMasterOnlyNode();
        final List<String> dataNodes = internalCluster().startDataOnlyNodes(2);
        final String index1 = "index-1";
        final String index2 = "index-2";
        createIndexWithContent(index1, dataNodes.get(0), dataNodes.get(1));
        createIndexWithContent(index2, dataNodes.get(1), dataNodes.get(0));
        final String repository = "test-repo";
        createRepository(repository, "mock");
        final String snapshotToDelete = "snapshot-to-delete";
        createFullSnapshot(repository, snapshotToDelete);
        final String cloneSource = "source-snapshot";
        createFullSnapshot(repository, cloneSource);
        internalCluster().stopNode(dataNodes.get(0));
        blockMasterOnWriteIndexFile(repository);
        final ActionFuture<AcknowledgedResponse> deleteFuture = clusterAdmin().prepareDeleteSnapshot(repository, snapshotToDelete)
            .execute();
        awaitNDeletionsInProgress(1);
        final ActionFuture<CreateSnapshotResponse> snapshot1 = startFullSnapshot(repository, "snapshot-1", true);
        awaitNumberOfSnapshotsInProgress(1);
        final ActionFuture<AcknowledgedResponse> cloneFuture = clusterAdmin().prepareCloneSnapshot(
            repository,
            cloneSource,
            "target-snapshot"