Ask HN: What gives with multiple submissions?

I've had a few submissions lately go through that were duplicates. I used to never check HN before submitting because it caught all dupes and treated it as an upvote. I just submitted "Ewww, you use PHP?" after reading, only to be surprised to be the first to submit it, followed by surprised again to see the @newsyc100 tweeting it had > 100 upvotes already. Checked the front page, and sure enough there's the article I just submitted as new, submitted 5 hours ago.

Has anything changed or am I just happening upon a quirk in the system a lot more frequently?