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
assertThat(thirdSnapshotResponse.get().getSnapshotInfo().state(), is(SnapshotState.FAILED));
        logger.info("--> verify both deletes have completed");
        assertAcked(deleteSnapshotsResponse.get());
        assertAcked(allDeletedResponse.get());
        logger.info("--> verify that all snapshots are gone");
        assertThat(client().admin().cluster().prepareGetSnapshots(repoName).get().getSnapshots(), empty());
    }
    public void testMasterFailOverWithQueuedDeletes() throws Exception {
        internalCluster().startMasterOnlyNodes(3);
        final String dataNode = internalCluster().startDataOnlyNode();
        final String repoName = "test-repo";
        createRepository(repoName, "mock");
        final String firstIndex = "index-one";
        createIndexWithContent(firstIndex);
        final String firstSnapshot = "snapshot-one";
        blockDataNode(repoName, dataNode);
        final ActionFuture<CreateSnapshotResponse> firstSnapshotResponse = startFullSnapshotFromNonMasterClient(repoName, firstSnapshot);
        waitForBlock(dataNode, repoName);
        final String dataNode2 = internalCluster().startDataOnlyNode();
        ensureStableCluster(5);
        final String secondIndex = "index-two";
        createIndexWithContent(secondIndex, dataNode2, dataNode);
        final String secondSnapshot = "snapshot-two";
        final ActionFuture<CreateSnapshotResponse> secondSnapshotResponse = startFullSnapshot(repoName, secondSnapshot);
        logger.info("--> wait for snapshot on second data node to finish");
        awaitClusterState(state -> {
            final SnapshotsInProgress snapshotsInProgress = state.custom(SnapshotsInProgress.TYPE, SnapshotsInProgress.EMPTY);
            return snapshotsInProgress.count() == 2 && snapshotHasCompletedShard(repoName, secondSnapshot, snapshotsInProgress);
        });
        final ActionFuture<AcknowledgedResponse> firstDeleteFuture = startDeleteFromNonMasterClient(repoName, firstSnapshot);