How can I define unique attribute types in my own schema?

["Sadik Kumlali" <Sadik.Kumlali@telenity.com>]

Dear All,

I could not found a way of doing an attribute unique while defining my
own schema.

I need following attributes:

bookCode (unique)
bookName
bookDescription 

What I want is not to allow duplicate bookCodes. 

I thought that it might caused by attribute identifiers such as SYNTAX
and EQUALITY. However, if I try to insert same 'cn's for inetOrgPerson,
it does not allow me while it allowing me to insert same 'sn's. I found
that both of 'cn' and 'sn' are defined as 'SUP name'. So there is no
difference in definition. What mechanism enforces the OpenLDAP not to
allow duplicated 'cn's?

Thanks in advance

Ali Sadik