Can't make Item Name (item_name) a unique field! Is this bug or feature?

For Item doctype, I have tested in multiple instance it seem that although I have set item_name as Unique, it take no effect.

Note: name and item_code is unique key, no issue.

So, now, the only way to make item_name unique for me is a database level,

ADD UNIQUE (item_name);

Is this a possible bug?