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
createRepository(repoName, "mock", repoPath);
        createRepository(otherRepoName, "mock");
        createIndexWithContent("index-one");
        createNSnapshots(repoName, randomIntBetween(2, 5));
        final int countOtherRepo = randomIntBetween(2, 5);
        createNSnapshots(otherRepoName, countOtherRepo);
        corruptIndexN(repoPath, getRepositoryData(repoName).getGenId());
        blockMasterFromFinalizingSnapshotOnIndexFile(repoName);
        blockMasterFromFinalizingSnapshotOnIndexFile(otherRepoName);
        client().admin().cluster().prepareCreateSnapshot(repoName, "snapshot-blocked-1").setWaitForCompletion(false).get();
        client().admin().cluster().prepareCreateSnapshot(repoName, "snapshot-blocked-2").setWaitForCompletion(false).get();
        client().admin().cluster().prepareCreateSnapshot(otherRepoName, "snapshot-other-blocked-1").setWaitForCompletion(false).get();
        client().admin().cluster().prepareCreateSnapshot(otherRepoName, "snapshot-other-blocked-2").setWaitForCompletion(false).get();
        awaitNumberOfSnapshotsInProgress(4);
        final String initialMaster = internalCluster().getMasterName();
        waitForBlock(initialMaster, repoName);
        waitForBlock(initialMaster, otherRepoName);
        internalCluster().stopCurrentMasterNode();
        ensureStableCluster(3, dataNode);
        awaitNoMoreRunningOperations();
        final RepositoryData repositoryData = getRepositoryData(otherRepoName);
        assertThat(repositoryData.getSnapshotIds(), hasSize(countOtherRepo + 2));
    }
    public void testConcurrentOperationsLimit() throws Exception {
        final String masterName = internalCluster().startMasterOnlyNode();
        internalCluster().startDataOnlyNode();
        final String repoName = "test-repo";
        createRepository(repoName, "mock");
        createIndexWithContent("index-test");
        final int limitToTest = randomIntBetween(1, 3);