@Override
                    public int compare(Value v1, Value v2)
                    {
                        if (v1.number == v2.number)
                        {
                            throw err("The enum value " +
                                    v2.enumGroup.getName() + "." + v2.getName() +
                                    " cannot have the same number as " +
                                    v1.enumGroup.getName() + "." + v1.getName(),
                                    v2.enumGroup.getProto());
                        }

                        return v1.number - v2.number;
                    }
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