Today a customer can run the following command to defragment a collection on a collection that is already being defragmented.

db.adminCommand({configureCollectionBalancing: "<database>.<collection>",  defragmentCollection: true}) 

This behavior is unintended and could allow a customer to input a different chunkSize (example below)

db.adminCommand({configureCollectionBalancing: "<database>.<collection>", defragmentCollection: true, chunkSize: 64})  

By altering the chunkSize on a collection that is already being defragmented could lead to improper or less than ideal behavior. So a customer should not be able to successfully run a command to start defragmentation on a collection that is being actively defragmented.