code: private void verifyCustomOptionNameUnique(final CustomOptionContext ctx) {
      if (ctx.customOptionName().customOptionNamePart().size() == 1) {
        final CustomOptionNamePartContext namePart = ctx.customOptionName().customOptionNamePart(0);
        verifyOptionNameUnique(namePart.getText(), namePart.getStart());
      }
    }
docs: Verifies uniqueness for custom options with a single NamePart. The multipart custom options
  should be validated during UninterpetedOption resolution in OptionsBuilder, not here!