code: def find_duplicate_schedule_items(all_items):
      
      duplicates = []
      seen_talks = {}
      for item in all_items:
          if item.talk and item.talk in seen_talks:
              duplicates.append(item)
              if seen_talks[item.talk] not in duplicates:
                  duplicates.append(seen_talks[item.talk])
          else:
              seen_talks[item.talk] = item
          # We currently allow duplicate pages for cases were we need disjoint
          # schedule items, like multiple open space sessions on different
          # days and similar cases. This may be revisited later
      return duplicates
docs: Find talks / pages assigned to mulitple schedule items