This one is via reddit.
It’s from the RSS feed itself. I do my best to weed out duplicates, going so far as to check the content of the last N stories and seeing if any of them match, even if the story_guid has changed (the globally unique ID that is not *supposed* to change.)
Not all RSS feeds are created equally. Some just don’t conform to the standard and pull things like this. If you give me the feed address, I’d be happy to take a look. (Is it just reddit.com straight?)