I have found - although I only tried it for one person - that it is easier, or at least fewer 'clicks', to delete the duplicates using FTM on my Mac, than to do it on Ancestry.
If you go to view your Media you can see a list of icons that gives you an indication of what's duplicated, and shows to whom the various files are attached. I used that dialog box to attach the record to all the people I wanted, then deleted the duplicate records.
I DON'T know whether the next sync will then delete the duplicates on Ancestry. I haven't tried that yet.
Still, I have a big tree and it's way too time-consuming and I am hoping for a global fix, if it ever comes.