web: sort a copy of the occurrences in DateList

web/src/components/events/DateList.tsx

@@ -65,7 +65,7 @@ export const DateList = ({ event }: { event: EventFragment }) => {
 
   let visibleOccurrences = [];
   if (oneOff || showAllDates || event.occurrences.length <= 4) {
-    visibleOccurrences = event.occurrences.sort((a, b) =>
+    visibleOccurrences = [...event.occurrences].sort((a, b) =>
       compareDates(a.start, b.start)
     );
   } else {