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
dataNode
        );
        final String snapshotTwo = "second-snapshot";
        final ActionFuture<CreateSnapshotResponse> createSnapshot2Future = startFullSnapshot(repoName, snapshotTwo);
        awaitNumberOfSnapshotsInProgress(2);
        assertAcked(startDeleteSnapshot(repoName, snapshotTwo).get());
        final SnapshotException sne = expectThrows(SnapshotException.class, createSnapshot2Future::actionGet);
        assertFalse(createSnapshot1Future.isDone());
        unblockNode(repoName, dataNode);
        assertSuccessful(createSnapshot1Future);
        assertThat(getRepositoryData(repoName).getGenId(), is(0L));
    }
    public void testStartWithSuccessfulShardSnapshotPendingFinalization() throws Exception {
        final String masterName = internalCluster().startMasterOnlyNode();
        final String dataNode = internalCluster().startDataOnlyNode();
        final String repoName = "test-repo";
        createRepository(repoName, "mock");
        createIndexWithContent("test-idx");
        createFullSnapshot(repoName, "first-snapshot");
        blockMasterOnWriteIndexFile(repoName);
        final ActionFuture<CreateSnapshotResponse> blockedSnapshot = startFullSnapshot(repoName, "snap-blocked");
        waitForBlock(masterName, repoName);
        awaitNumberOfSnapshotsInProgress(1);
        blockNodeOnAnyFiles(repoName, dataNode);
        final ActionFuture<CreateSnapshotResponse> otherSnapshot = startFullSnapshot(repoName, "other-snapshot");
        awaitNumberOfSnapshotsInProgress(2);
        assertFalse(blockedSnapshot.isDone());
        unblockNode(repoName, masterName);
        awaitNumberOfSnapshotsInProgress(1);
        awaitMasterFinishRepoOperations();