[TrainedMonkey]

I call birthday paradox: https://en.wikipedia.org/wiki/Birthday_problem

Number of all possible topics is N, number of news on FP is 30. For example if N is 365, you would expect 70% chance that two separate items on FP mention the same topic.

Although, HN recurrence probably a bigger contributor. When a person gets reminded of some previous highly upvoted link and re-submits it. The highly upvoted link gets upvoted again and now you have two items on FP with similar keywords.

[saagarjha]

This only works if your sample space is small, whereas I'd argue the range of topics that can show up on Hacker News is nothing but ;)

[wpasc]

I agree, but the range of topics which may be popular at any given time is narrower and two stories about a single topic are likely to trend at similar times shrinking the sample space somewhat.