Thanks @AlexisJazz - there is an issue of re-sending notification (a curly bracket in this case), but I could not find the exact steps to reproduce the issue you describe (two identical notification for the same issue). It'd be great if you can provide a screenshot of the Special:Notification page or the drop-down Notification flyout to show the duplicate.

The issue confirmed in betalabs is more about a wrong mention authorship attribution than duplicate notification- the steps are a bit tricky:

1. User ET17 mentions user ET18 with incorrect entry (one curly bracket is omitted):

: Thanks, {u|ET18}} ~~~~

2. User ET19 notices the error and corrects it and also adds his own mention of ET18:

:: {{ping|ET18}} ~~~~
So, the resulting edit made by ET19 looks as the following - https://en.wikipedia.beta.wmflabs.org/w/index.php?title=Zilant22&oldid=prev&diff=445121

3. Now, user ET18 receives two identical mentions (almost) from ET19 - the fact that user`ET17` has authored the first mention is lost :

 [enwiki]> select event_id, event_type, event_agent_id, event_extra, user_name from echo_event join user on user_id=event_agent_id  where event_id in (select notification_event  from echo_notification where notification_user=6914  and notification_timestamp>20200813000000) and event_type='mention'\G
*************************** 1. row ***************************
      event_id: 880630
    event_type: mention
event_agent_id: 6915
   event_extra: a:4:{s:7:"content";s:20:": Thanks, {{u|ET18}}";s:13:"section-title";b:0;s:5:"revid";i:445121;s:15:"mentioned-users";a:1:{i:6914;i:6914;}}
     user_name: ET19
*************************** 2. row ***************************
      event_id: 880632
    event_type: mention
event_agent_id: 6915
   event_extra: a:4:{s:7:"content";s:16:":: {{ping|ET18}}";s:13:"section-title";b:0;s:5:"revid";i:445121;s:15:"mentioned-users";a:1:{i:6914;i:6914;}}
     user_name: ET19
2 rows in set (0.00 sec)

In T259532#6384651, @Etonkovidova wrote:

Thanks @AlexisJazz - there is an issue of re-sending notification (a curly bracket in this case), but I could not find the exact steps to reproduce the issue you describe (two identical notification for the same issue). It'd be great if you can provide a screenshot of the Special:Notification page or the drop-down Notification flyout to show the duplicate.

Better late than never, eh?

See https://en.wikipedia.org/wiki/Wikipedia:Administrators%27_noticeboard/IncidentArchive1045#Aspersions_at_ARS. The "Thanks, Alexis Jazz, and I will take a look." was written by Sitush, not Narky Blert.

Link targets:

• https://en.wikipedia.org/wiki/Wikipedia:Administrators%27_noticeboard/Incidents?markasread=196685883&markasreadwiki=enwiki#Aspersions_at_ARS
• https://en.wikipedia.org/wiki/Wikipedia:Administrators%27_noticeboard/Incidents?markasread=196685885&markasreadwiki=enwiki#Aspersions_at_ARS