Neither... I don't want that the same value for a given attribute can be assigned to more than one entry. That is, if I set mailalternateaddress=foo@domain.com to entry dn=user1,o=domain.com,dc=myisp , the DS should prevent the assignment of the same mailalternateaddress to dn=user2,o=domain.com,dc=myisp .
Moreover, I have a particular situation in which I would also like to prevent the duplication of those attributes in the whole rootdn (eg foo@domain.com should be negated to dn=user3,o=otherdomain.com,dc=myisp too).