This came out of HELP-60709 where a user had to repeatedly manually decrease the number of initial chunks to get an acceptable number of split points to continue resharding. Currently, when we sample for the initial split points, we only deduplicate documents selected by _id. Maybe we could consider deduplicating by the new shard key in order to allow customers with enough unique shard keys to proceed with resharding. 

The risk of de-duplicating by shard key however, is that the split points chosen may not lead to a good chunk distribution I.E. if one shard key is far more frequent than the others.