code: private boolean alreadyReported(int stackPos) {
          if (stack.getStackDepth() <= stackPos) {
              return false;
          }
  
          OpcodeStack.Item item = stack.getStackItem(stackPos);
          XField field = item.getXField();
          if (field == null) {
              return false;
          }
  
          String fieldName = field.getName();
          return !checkedFields.add(fieldName);
      }
docs: returns whether the collection has already been reported on
  
  @param stackPos
  the position on the opstack to check
  
  @return whether the collection has already been reported.