code: def find_clashes(all_items):
      
      clashes = {}
      seen_venue_slots = {}
      for item in all_items:
          for slot in item.slots.all():
              pos = (item.venue, slot)
              if pos in seen_venue_slots:
                  if seen_venue_slots[pos] not in clashes:
                      clashes[pos] = [seen_venue_slots[pos]]
                  clashes[pos].append(item)
              else:
                  seen_venue_slots[pos] = item
      # We return a list, to match other validators
      return clashes.items()
docs: Find schedule items which clash (common slot and venue)