--- lib/CalDAV/Reminder/Backend.php.orig	2021-02-19 09:49:36.000000000 +0100
+++ lib/CalDAV/Reminder/Backend.php	2021-05-06 10:56:17.252243326 +0200
@@ -70,5 +70,6 @@
 			->where($query->expr()->lte('cr.notification_date', $query->createNamedParameter($this->timeFactory->getTime())))
 			->leftJoin('cr', 'calendarobjects', 'co', $query->expr()->eq('cr.object_id', 'co.id'))
-			->leftJoin('cr', 'calendars', 'c', $query->expr()->eq('cr.calendar_id', 'c.id'));
+			->leftJoin('cr', 'calendars', 'c', $query->expr()->eq('cr.calendar_id', 'c.id'))
+			->groupBy(['cr.event_hash','cr.type','cr.uid']);
 		$stmt = $query->execute();