Sorry, but I have to bump and add a little to this.
The file converter does the remove files step after the batch completes, which is fair enough.
The problem is that if something breaks during the run then this never happens so you end up with the originals plus whatever was converted.
With this in mind, could there be a pause button or one that skips any remaining in the queue and treats the ones that have finished as normal (IE deletes the old file)
I've started batches and not noticed I dont have the disk space for it to complete half way through a large queue so cancelling means I have to unpick the duplication and start again or start getting creative on freeing space another way.
Alternatively, could the queue be split by the folders involved, rather than one big list. That way if everything else stays as is any folders involved will have been completed so if there is an issue they arent left with 2 sets of files.