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 blockedRepoName = "test-repo-blocked";
        final String otherRepoName = "test-repo";
        createRepository(blockedRepoName, "mock");
        createRepository(otherRepoName, "fs");
        createIndexWithContent("test-index");
        createFullSnapshot(blockedRepoName, "blocked-snapshot");
        blockNodeOnAnyFiles(blockedRepoName, masterNode);
        final ActionFuture<AcknowledgedResponse> slowDeleteFuture = startDeleteSnapshot(blockedRepoName, "*");
        logger.info("--> waiting for concurrent snapshot(s) to finish");
        createNSnapshots(otherRepoName, randomIntBetween(1, 5));
        assertAcked(startDeleteSnapshot(otherRepoName, "*").get());
        unblockNode(blockedRepoName, masterNode);
        assertAcked(slowDeleteFuture.actionGet());
    }
    public void testSnapshotRunsAfterInProgressDelete() throws Exception {
        final String masterNode = internalCluster().startMasterOnlyNode();
        internalCluster().startDataOnlyNode();
        final String repoName = "test-repo";
        createRepository(repoName, "mock");
        ensureGreen();
        createIndexWithContent("index-test");
        final String firstSnapshot = "first-snapshot";
        createFullSnapshot(repoName, firstSnapshot);
        blockMasterFromFinalizingSnapshotOnIndexFile(repoName);
        final ActionFuture<AcknowledgedResponse> deleteFuture = startDeleteSnapshot(repoName, firstSnapshot);
        waitForBlock(masterNode, repoName);
        final ActionFuture<CreateSnapshotResponse> snapshotFuture = startFullSnapshot(repoName, "second-snapshot");
        unblockNode(repoName, masterNode);
        final UncategorizedExecutionException ex = expectThrows(UncategorizedExecutionException.class, deleteFuture::actionGet);
        assertThat(ex.getRootCause(), instanceOf(IOException.class));