I noticed that if I open the notifications dropdown immediately after a full page refresh, I see items twice. But then after closing and opening it I see the item only once. Unfortunately I can't upload a video screen recording (nudge nudge) but here are some still frames from the video:
Initial state, before clicking the dropdown.
After clicking, the loading placeholder appears.
After a moment one result shows.
❌ After another moment a second result shows.
Then I click the button to dismiss the popup.
I click it again to open, and see the loading placeholder again.
After a moment one result appears.
✅ Waiting longer, no other items appear.
Thanks for the detailed report! I've managed to reproduce it but only flakily, and only in deployed environments. I'll have a good look once I'm back in the office.