Take a look to the picture.
I have some Podcast RSS feeds from IVOOX that nobody knows why, use to generate new entries of old items. Same happens with other feeds.
Activating this smart rule sends to Trash all new duplicate added entries. The rule is run on synchronization, and if Sync is set to automatic, after each feeds synchronization, database is synchronized as well and the feeds are removed.
At least is working to me this evening.
EDIT: Thanks to @cgrunenberg, there is an event called “OnNews” that improves my approach.