When something fails and a caller retries, it can throw an error, when the chunk was splited:

errmsg: new split key { x: 10.0 } is a boundary key of existing chunk [{ x: 10.0 },{ x: MaxKey })

There was the issue https://jira.mongodb.org/browse/SERVER-51805 to make these calls idempotent, but was closed with "Won't fixed"

The proposal is at least to change the check of range boundaries during the splitting, that it does return false in case when the splitting already done.