I deleted everything after reading your previous post regarding your server resources. Afterward, I realized this breaks the ability of TT-RSS to suppress duplicates. I was trying to be helpful.
Thanks, but unless you want to annoy yourself with duplicate articles, don't do that in the future. Let the normal expiration process work.
Basically when you deleted everything, you got all the stuff still available in the feed back - because there was nothing to compare it to. Delete means delete, literally. This feature is mostly used for debugging, it's not really supposed to be readily exposed in the UI (which is an oversight on my part, I suppose, because it currently is).
Just about every type of feed EXCEPT current news could be set to 0 (purge immediately as an article is marked), so only unread articles would be stored on the server.
This would unfortunately cause an endless stream of annoying duplicates, just like what happened when you deleted all the articles. It's not like mail, news feeds have stuff available for a while, sometimes months at a time. If tt-rss can't find the article in the local database, it will re-add it and thus show you stuff you already read.
When using normal expiration process, there are some measures in place to prevent duplicates from showing even if the articles stay in the feed longer than the expiry timeout, but when you actually manually delete stuff, all bets are off.
So, my point is, being overly aggressive in deleting stuff is not always a good idea.