I’ve experienced the same thing and have been wondering if this is perhaps related to tasks being captured by MacOS and then replicated by Exchange. My eMail/calendar/tasks ultimately reside in MS Exchange.
Wondering if this issue of duplicate reminders was looked into? These days, I am just using iCloud sync (no more MS Outlook/Exchange etc) and yet, I am still experiencing duplicate reminders.
It seems that if I create a reminder using “\reminder” and I don’t modify anything about the reminder (e.g. date, time, reminder list), then everything is fine and only a single reminder is created. I’m able to update the details in the reminder app and everything is fine.
However, if I update any of the reminder details in the Agenda reminder dialog, then two reminders are always created in the Reminders app.
[Current setup: Agenda 19.2.1 (315) running on MacOS Sequoia 15.0]
Thank you Drew for your quick response and ideas. Although I did experience this problem repeatedly, I now suspect it was a MacOS configuration problem - having done a clean install of the OS the problem seems to have stopped. Apologies.