unique overlay

Dear Expert, I am currently using unique overlay with uid and mail attributes which is working perfectly for me but now i want to make it unique across the directory like if i am using mailAlternateAddress attribute and try to add any value which is already defined in mail attribute should not be allowed.

Example -- mail: piyush@xxx.com is already there in ldap dit then piyush@xxx.com should not be allowed in mailAlternateAddress attribute's value and vice versa.

Any help would be appreciated !!!

OpenLDAP version - 2.4.19 BDB version - 4.7.25