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(blockedRepoName, "mock");
        createRepository(otherRepoName, "fs");
        createIndex("foo");
        ensureGreen();
        createIndexWithContent("index-slow");
        final ActionFuture<CreateSnapshotResponse> createSlowFuture = startAndBlockFailingFullSnapshot(blockedRepoName, "blocked-snapshot");
        client().admin()
            .cluster()
            .prepareCreateSnapshot(otherRepoName, "snapshot")
            .setIndices("does-not-exist-*")
            .setWaitForCompletion(false)
            .get();
        unblockNode(blockedRepoName, internalCluster().getMasterName());
        expectThrows(SnapshotException.class, createSlowFuture::actionGet);
        assertBusy(() -> assertThat(currentSnapshots(otherRepoName), empty()), 30L, TimeUnit.SECONDS);
    }
    public void testMultipleReposAreIndependent() throws Exception {
        internalCluster().startMasterOnlyNode();
        
        
        final String dataNode = startDataNodeWithLargeSnapshotPool();
        final String blockedRepoName = "test-repo-blocked";
        final String otherRepoName = "test-repo";
        createRepository(blockedRepoName, "mock");
        createRepository(otherRepoName, "fs");
        createIndexWithContent("test-index");
        final ActionFuture<CreateSnapshotResponse> createSlowFuture = startFullSnapshotBlockedOnDataNode(
            "blocked-snapshot",
            blockedRepoName,
            dataNode