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(repoName, dataNode);
        assertSuccessful(createFastSnapshot);
        assertSuccessful(createSlowFuture);
        final RepositoryData repositoryData = getRepositoryData(repoName);
        assertThat(repositoryData.shardGenerations(), is(ShardGenerations.EMPTY));
    }
    public void testQueuedDeleteAfterFinalizationFailure() throws Exception {
        final String masterNode = internalCluster().startMasterOnlyNode();
        final String repoName = "test-repo";
        createRepository(repoName, "mock");
        blockMasterFromFinalizingSnapshotOnIndexFile(repoName);
        final String snapshotName = "snap-1";
        final ActionFuture<CreateSnapshotResponse> snapshotFuture = startFullSnapshot(repoName, snapshotName);
        waitForBlock(masterNode, repoName);
        final ActionFuture<AcknowledgedResponse> deleteFuture = startDeleteSnapshot(repoName, snapshotName);
        awaitNDeletionsInProgress(1);
        unblockNode(repoName, masterNode);
        assertAcked(deleteFuture.get());
        final SnapshotException sne = expectThrows(SnapshotException.class, snapshotFuture::actionGet);
        assertThat(sne.getCause().getMessage(), containsString("exception after block"));
    }
    public void testAbortNotStartedSnapshotWithoutIO() throws Exception {
        internalCluster().startMasterOnlyNode();
        final String dataNode = internalCluster().startDataOnlyNode();
        final String repoName = "test-repo";
        createRepository(repoName, "mock");
        createIndexWithContent("test-index");
        final ActionFuture<CreateSnapshotResponse> createSnapshot1Future = startFullSnapshotBlockedOnDataNode(
            "first-snapshot",
            repoName,