How to do unique users

I have the following tables (mucho simplified):

usrs{
serial usr_id,
var(54) username);

addrs(
serial addr_id,
var(54) addr};

usr_addr_type(
serial usr_addr_type_id,
var(16) type -- one of which is 'primary'};

usr_addrs{
int4 usr_id, --FK to usrs table
int4 addr_id, -- FK to addrs table
int4 usr_addr_type_id -- FK to usr_addr_type table);

================================
My problem: The usrs can have as many addresses as there are types. The
addresses are also used for 'org_addrs' in combination with 'org_addr_types'.
(This explains the 'usr_addrs' table)

I want to EFFECTIVELY do a unique index on:

'usrs.username' their 'primary' address in usr_addrs'. How would I have many
addresses in 'usr_addrs' for a particular address, but one of them used for
uniqueness of the of the 'usr'
?

---------------------------------------------
This message was sent using CVC Internet's MailMan.
http://www.cvc.net