I don't believe this is an actual feature, but I've noticed a trend that I've been calling "twos", where, at any given moment, a particular subject (present as a word or phrase in the article's title) will show up twice on the front page.
Funnily enough, currently the "twos" subject is "Hacker News", with this article and "What I've Learned from Hacker News" both being present in the front page at the same time.
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.
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.
That's how I interpret it as well. I also think interesting articles on a topic jog people's memory and cause them to re-submit interesting articles from the past on that topic.
Funnily enough, currently the "twos" subject is "Hacker News", with this article and "What I've Learned from Hacker News" both being present in the front page at the same time.