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 List<String> snapshotNames = createNSnapshots(repoName, limitToTest + 1);
        assertAcked(
            client().admin()
                .cluster()
                .prepareUpdateSettings()
                .setPersistentSettings(
                    Settings.builder().put(SnapshotsService.MAX_CONCURRENT_SNAPSHOT_OPERATIONS_SETTING.getKey(), limitToTest).build()
                )
                .get()
        );
        blockNodeOnAnyFiles(repoName, masterName);
        int blockedSnapshots = 0;
        final List<ActionFuture<CreateSnapshotResponse>> snapshotFutures = new ArrayList<>();
        ActionFuture<AcknowledgedResponse> deleteFuture = null;
        for (int i = 0; i < limitToTest; ++i) {
            if (deleteFuture != null || randomBoolean()) {
                snapshotFutures.add(startFullSnapshot(repoName, "snap-" + i));
                ++blockedSnapshots;
            } else {
                deleteFuture = startDeleteSnapshot(repoName, randomFrom(snapshotNames));
            }
        }
        awaitNumberOfSnapshotsInProgress(blockedSnapshots);
        if (deleteFuture != null) {
            awaitNDeletionsInProgress(1);
        }
        waitForBlock(masterName, repoName);
        final ConcurrentSnapshotExecutionException cse = expectThrows(
            ConcurrentSnapshotExecutionException.class,
            () -> client().admin().cluster().prepareCreateSnapshot(repoName, "expected-to-fail").execute().actionGet()