This is almost always due to a problem with the feed itself, and more often than not it's because the feed's publisher is incorrectly changing the publication date of existing items, making RSS readers like FeedDemon think they're new items.
Speaking more technically, when an RSS reader downloads a feed, it has to figure out which items in that feed it already knows about and which ones are new. Many feeds uniquely identify each item, so usually this isn't a big deal. But some feeds don't uniquely identify items, so it's up to the software to figure out how to uniquely ID each item.
An item's publication date is part of this unique ID, so if a feed changes the publication date of existing items, you'll end up seeing duplication of those items in your RSS reader.
