make calendar event item keys unique

web/src/components/events/EventContainer.tsx

@@ -224,7 +224,12 @@ const CalendarDay = ({
     <h3>{formatDate(parse(day, "yyyy-MM-dd", new Date()), "eeee dd.MM.")}</h3>
     <ul>
       {events.map((event) => (
-        <EventItem key={event.id} event={event} mode="calendar" size="small" />
+        <EventItem
+          key={`${day}--${event.id}--${event.occurrence.id}`}
+          event={event}
+          mode="calendar"
+          size="small"
+        />
       ))}
     </ul>
   </div>