Why a synced copy alerts you twice, which Reminders setting stops it, and why the one that sounds safest is the one causing it.
Set Reminders to Off on the sync. It's in Appearance, and Off is the default for new syncs. Copies then carry no alerts of their own, and you get one notification from the original event instead of two.
The copy is a real event on the destination calendar. If it carries reminders, your phone treats it as its own thing to notify you about, and it has no idea it's a copy of something you're already being reminded about. Two calendars, two alerts, same meeting.
Three settings, and what each does:
Off puts no reminders on the copy. Nothing fires from the destination calendar.
Calendar default leaves the copy's reminders unset, so the destination calendar's own default applies. Most people's calendars have a default of 10 or 30 minutes, so this usually produces the duplicate rather than avoiding it. It's the right choice only when the destination calendar is the one you actually watch and the source is the one you don't.
Match original carries the original's reminders across. Useful when you want alerts on a work phone for events living on a personal account and you don't get notifications from that account at all.
Existing copies keep the reminders they were created with. The setting applies to what the sync writes next, so open the sync's menu and choose Re-sync to rewrite the copies that already exist.
If they still fire after that, check whether the reminder is coming from the calendar rather than the event. Google's "Notifications for this calendar" and Outlook's default reminder apply to everything in the calendar, and they sit outside anything a sync can control. A copy with no reminders of its own will still trigger a calendar-wide default in Outlook. Turning that off is a change in your calendar app, not here.
Also worth checking: two syncs writing into the same calendar. If you built one sync per source and pointed them all at the same destination, each one is writing its own copy, and Reminders is set per sync.
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