I use Rojo.com for my RSS feeds and I will often get duplicate entries. Sometimes where there was a spelling correction/change in the title.
But sometimes I'd get several of the same entries from one source, usually Joystiq, like they don't post enough already. It's terrible to have 100 new articles but each one from Joystiq has seven duplicates.
I don't have a solution. I'm just letting you know that it isn't limited to FeedDemon.