Hi, Tejesh.
I provided you answer in your previous ticket. Basically, there are 2 levels where this limitation is enforced: oxTrust (web UI) and OpenDJ (LDAP).
Upgrading should have removed the later (at least it's not present in freshly installed 3.1.6; if upgrade script doesn't lift it, then, as Zico says, please try to do a bit of research on OpenDJ's "dsconfig" features, you need to find out how to use it to remove uniqueness from "mail" attribute), and the property I mentioned last time should remove the former.