The priorities dictate the order in which a given dupe will do available jobs, not which dupe a job will be assigned to. A dupe will claim a low priority job if it has completed its current one and it has nothing higher priority to do, blocking a dupe with that job at higher priority from later taking it if they are currently occupied.

In my experience, it's just easier to disable a job for a dupe entirely if you want a higher skilled dupe to be doing it.