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.
test/framework/src/main/java/org/elasticsearch/snapshots/AbstractSnapshotIntegTestCase.java
protected void addBwCFailedSnapshot(String repoName, String snapshotName, Map<String, Object> metadata) throws Exception {
        final ClusterState state = clusterAdmin().prepareState().get().getState();
        final RepositoriesMetadata repositoriesMetadata = state.metadata().custom(RepositoriesMetadata.TYPE);
        assertNotNull(repositoriesMetadata);
        final RepositoryMetadata initialRepoMetadata = repositoriesMetadata.repository(repoName);
        assertNotNull(initialRepoMetadata);
        assertThat(
            "We can only manually insert a snapshot into a repository that does not have a generation tracked in the CS",
            initialRepoMetadata.generation(),
            is(RepositoryData.UNKNOWN_REPO_GEN)
        );
        final Repository repo = getRepositoryOnMaster(repoName);
        final SnapshotId snapshotId = new SnapshotId(snapshotName, UUIDs.randomBase64UUID(random()));
        logger.info("--> adding old version FAILED snapshot [{}] to repository [{}]", snapshotId, repoName);
        final SnapshotInfo snapshotInfo = new SnapshotInfo(
            new Snapshot(repoName, snapshotId),
            Collections.emptyList(),
            Collections.emptyList(),
            Collections.emptyList(),
            "failed on purpose",
            SnapshotsService.OLD_SNAPSHOT_FORMAT,
            0L,
            0L,
            0,
            0,
            Collections.emptyList(),
            randomBoolean(),
            metadata,
            SnapshotState.FAILED,
            Collections.emptyMap()