If you started your postgrad study prior to 2007 and have a "Departmental-pg" usercode, you will likely also have a "student" usercode.
Each usercode has an associated mailbox. Your "student" usercode probably already has a new webmail mailbox of the form Joe.Bloggs@lincolnuni.ac.nz, so the webmail mailbox for your "departmental-pg" usercode will have a name like Joe.Bloggs2@lincolnuni.ac.nz
You must read all the email which arrives from time to time in each of these mailboxes. One technique for managing this situation is to forward incoming mail from multiple mailboxes to a single mailbox, using forwarding rules.
If you wish to hide one of your email addresses in the AddressBook please email email@example.com.