Generally if you get a NotMaster error back from an attempted write you know that nothing was written. With SERVER-24574, however, it's possible for a node that steps down mid-write to actually return NotMaster to the client. When we write to the config servers, we retry non-idempotent writes on NotMaster errors because we assume no write actually happened, but that assumption is now invalid, so the retries can lead applying a non-idempotent write multiple times.