Exact dupes from the same source are easy if they come in succession, but some sites have got their postings on on schedules with tools like Hootsuite, It's easy for them to space postings out enough so that once I've read a feed I'm sure to see the exact same story 8~12 hours later. That's probably necessary for SNS like Twitter where your post can get lost in the flow, but I wish they'd keep it out of their RSS.