how to ensure uniquesness of attribute?

["S.T. Wong" <st@hp735c.csc.cuhk.edu.hk>]

Hi there,

I'd like to know if it's possible to check updates of particular attributes,
e.g. mailacceptinggeneralid, so that the value of these attributes are unique 
within the database.   Would anyone please help ?

Thanks a lot!
Regards,
ST Wong (st-wong@cuhk.edu.hk)