use unique keys for upcoming event items

web/src/components/events/UpcomingEvents.tsx

@@ -18,7 +18,7 @@ export const UpcomingEvents = ({ events }: { events: EventFragment[] }) => {
       <ul className={styles.eventList}>
         {upcomingSingularEvents.slice(0, 10).map((event) => (
           <EventItem
-            key={event.id}
+            key={`${event.id}-${event.occurrence.id}`}
             event={event}
             mode="singular"
             size="medium"