It is possible to have multiple threads running the same moveChunk command at the same time on the same shard. Only one of which is doing the actual work while the rest waits for it to finish. The waitForMoveChunkStep assumes that there can only be one moveChunk and returns right away instead of checking the other ops:

https://github.com/mongodb/mongo/blob/r3.4.0-rc0/jstests/libs/chunk_manipulation_util.js#L129