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
startAndBlockOnDeleteSnapshot(repoName, "*");
        client().admin().cluster().prepareCreateSnapshot(repoName, "snapshot-three").setWaitForCompletion(false).get();
        startDeleteSnapshot(repoName, "*");
        awaitNDeletionsInProgress(2);
        internalCluster().stopCurrentMasterNode();
        ensureStableCluster(3);
        awaitNoMoreRunningOperations();
    }
    public void testQueuedOperationsOnMasterDisconnect() throws Exception {
        internalCluster().startMasterOnlyNodes(3);
        final String dataNode = internalCluster().startDataOnlyNode();
        final String repoName = "test-repo";
        createRepository(repoName, "mock");
        createIndexWithContent("index-one");
        createNSnapshots(repoName, randomIntBetween(2, 5));
        final String masterNode = internalCluster().getMasterName();
        final NetworkDisruption networkDisruption = isolateMasterDisruption(NetworkDisruption.DISCONNECT);
        internalCluster().setDisruptionScheme(networkDisruption);
        blockNodeOnAnyFiles(repoName, masterNode);
        ActionFuture<AcknowledgedResponse> firstDeleteFuture = client(masterNode).admin()
            .cluster()
            .prepareDeleteSnapshot(repoName, "*")
            .execute();
        waitForBlock(masterNode, repoName);
        final ActionFuture<CreateSnapshotResponse> createThirdSnapshot = client(masterNode).admin()
            .cluster()
            .prepareCreateSnapshot(repoName, "snapshot-three")
            .setWaitForCompletion(true)
            .execute();
        awaitNumberOfSnapshotsInProgress(1);