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
assertThat(client().prepareSearch(testIndex).setSize(0).get().getHits().getTotalHits().value, equalTo(100L));
        logger.info("--> start relocations");
        allowNodes(testIndex, 1);
        logger.info("--> wait for relocations to start");
        assertBusy(
            () -> assertThat(
                client().admin().cluster().prepareHealth(testIndex).execute().actionGet().getRelocatingShards(),
                greaterThan(0)
            ),
            1L,
            TimeUnit.MINUTES
        );
        logger.info("--> start two snapshots");
        final String snapshotOne = "snap-1";
        final String snapshotTwo = "snap-2";
        final ActionFuture<CreateSnapshotResponse> snapOneResponse = client().admin()
            .cluster()
            .prepareCreateSnapshot(repoName, snapshotOne)
            .setWaitForCompletion(false)
            .setIndices(testIndex)
            .execute();
        final ActionFuture<CreateSnapshotResponse> snapTwoResponse = client().admin()
            .cluster()
            .prepareCreateSnapshot(repoName, snapshotTwo)
            .setWaitForCompletion(false)
            .setIndices(testIndex)
            .execute();
        snapOneResponse.get();
        snapTwoResponse.get();
        awaitNoMoreRunningOperations();