I'd like a way to remove/ auto-mark as read any duplicate feed based on content
Do you mean also in several feeds ?
Most client apps have this feature built in; therefore, I am removing my votes for this feature because it would be redundant.
I think this should happen auto-magically. I am subscribed myself to Heise (German magazin) feeds and general news are shared with the same title and content across all their feeds.
It would be nice if such duplicates could be detected automatically.