code: private Set<String> collidingFieldNames(ImmutableList<Field> fields) {
      Set<String> fieldNames = new LinkedHashSet<>();
      Set<String> collidingNames = new LinkedHashSet<>();
      for (Field field : fields) {
        if (!fieldNames.add(field.name())) {
          collidingNames.add(field.name());
        }
      }
      return collidingNames;
    }
docs: Returns the set of names that are not unique within {@code fields}.