ShardLocal is given a Shard::RetryPolicy::kIdempotent retry policy for the config.databases write, where WriteConcernError is a retryable error (see remote_command_retry_scheduler.cpp. However, it runs into a DuplicateKeyError when it retries the write. See BF-6713 for details of the failure.

I hypothesize that the issue is that ShardLocal/CatalogClient uses majority write/read concern, and instead it should use local and then mongos should call _configsvrEnableSharding with majority write concern so it waits after the write. Might want to make sure the mongos can deal with the WriteConcernError properly, so the config stepdown suite doesn't fail loudly.