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
internalCluster().startDataOnlyNode();
        final String repoName = "test-repo";
        createRepository(repoName, "mock");
        createIndexWithContent("index-one");
        createIndexWithContent("index-two");
        createNSnapshots(repoName, randomIntBetween(1, 5));
        final ActionFuture<AcknowledgedResponse> deleteFuture = startAndBlockOnDeleteSnapshot(repoName, "*");
        final ActionFuture<CreateSnapshotResponse> snapshotThree = startFullSnapshot(repoName, "snapshot-three", true);
        final ActionFuture<CreateSnapshotResponse> snapshotFour = startFullSnapshot(repoName, "snapshot-four", true);
        awaitNumberOfSnapshotsInProgress(2);
        assertAcked(client().admin().indices().prepareDelete("index-two"));
        unblockNode(repoName, masterNode);
        assertThat(snapshotThree.get().getSnapshotInfo().state(), is(SnapshotState.SUCCESS));
        assertThat(snapshotFour.get().getSnapshotInfo().state(), is(SnapshotState.SUCCESS));
        assertAcked(deleteFuture.get());
    }
    public void testQueuedSnapshotsWaitingForShardReady() throws Exception {
        internalCluster().startMasterOnlyNode();
        internalCluster().startDataOnlyNodes(2);
        final String repoName = "test-repo";
        createRepository(repoName, "fs");
        final String testIndex = "test-idx";
        
        assertAcked(prepareCreate(testIndex, 2, indexSettingsNoReplicas(between(2, 10))));
        ensureGreen(testIndex);
        logger.info("--> indexing some data");
        for (int i = 0; i < 100; i++) {
            indexDoc(testIndex, Integer.toString(i), "foo", "bar" + i);
        }
        refresh();