@Override
    public void putExtraOption(String key, Object value)
    {
        if (extraOptions.put(key, value) != null)
            throw err("Duplicate enum option: " + key, getProto());
    }
Disabled by default (the earlier protoc 2.x versions enabled this by default, but was changed later on).
protostuff-parser/src/main/java/io/protostuff/parser/EnumGroup.java