I was unable to reproduce. I used the settings you mentioned, then scrolled down until there were 320 entries, then I copy pasted the page into Microsoft Excel. Then I turned on Conditional Formatting -> Highlight Cell Rules -> Duplicate Values, which highlights duplicate cells red. Then I sorted A-Z and deleted any rows that weren't article titles or article descriptions to get rid of false positives. All 320 were unique.

Posted at https://en.wikipedia.org/wiki/Wikipedia_talk:New_pages_patrol/Reviewers#Special:NewPagesFeed_issues%3F to collect more information about whether this is common or uncommon.

Also suggested that a video be submitted to see if we can learn more steps to reproduce from that.

I found a second person who has this problem.

I've had the first issue even since I started reviewing for AfC some years ago. My theory is that if there's any sort of update in the list (e.g. brand new articles or articles being taken off the list) then all the entries just get repeated again from the top. I could be wrong. --Rambley

Sounds like we need to do some work on the steps to reproduce. Rambley's hypothesis is worth trying out: this bug may only happen when a new article is added to the queue, which can take a couple minutes in production, or may never happen when localhost testing unless you intentionally do it.

Someone pointed out that the test script https://en.wikipedia.org/wiki/User:JSherman_(WMF)/T414892.js exists to check Special:NewPagesFeed for duplicates. You install it, then click something in your toolbox to turn it on. Probably a bit more efficient than me copy pasting things into Excel and turning on conditional formatting -> highlight duplicates, then sorting.

cc @jsn.sherman who has worked on this style of bug before.