It duplicated pics, but not all. So strange.
Thanks for reporting back.
In my case, all the pictures are many months or even years old. Though I did not check super carefully in every subdirectory, for those I checked every picture was duplicated. Perhaps if there is greater variation in file dates, maybe newer ones that
have recently been backed up are not duplicated (?). Just a guess that Maintenance tries to establish some kind of safe daily baseline state which quickly becomes a storage hog if not purged. In any case, I agree it is so strange. Microsoft needs to fix
this bizarreness or explain their intent.
With File History (maintenance mode) disabled, I've been running hourly File History backups all day from two networked machines to a single external drive hanging off one of them. No duplicates and it should be impossible for Maintenance to interfere.
Now switched to 6 hour intervals.
BTW, Control Panel > File History > Files last copied on date/time is often incorrect with respect to scheduled runs, while time stamps on the actual File History data files are accurate. To check activity I created a Custom View in Event Viewer,
filtering to see all events for sources FileHistory-Core and FileHistory-Engine.