A notification for "6 new topics" led me to
https://www.mediawiki.org/w/index.php?title=Talk:Beta_Features/Hovercards&topiclist_sortby=newest&fromnotif=1#flow-post-s5zkrn9nzud1ownz
this view contains repeated topics.
I can reproduce the problem if I visit the link with a second account, but not whilst logged-out.
The &topiclist_sortby=newest seems to be the troublesome bit.