simulate multiple primary keys

I have the following table:

gms=> \d jobclock
Table "public.jobclock"
Column | Type | Modifiers
-------------+--------------------------------+----------------------------------------------------------------
jobclock_id | integer | not null default nextval('jobclock_jobclock_id_seq'::regclass)
employee_id | integer | not null
machine_id | character varying(4) | not null
workorder | character varying(8) | not null
operation | integer | not null
bartype | character varying(10) | not null
clockin | timestamp(0) without time zone | not null
clockout | timestamp(0) without time zone | default NULL::timestamp without time zone
comments | character varying(255) | default NULL::character varying
Indexes:
"jobclock_pkey" PRIMARY KEY, btree (jobclock_id)
...

I need to keep jobclock_id unique and not null, but I also need to
ensure that no row is duplicated. Is my best bet to drop the current
primary key and make a primary key out of the columns that I want to
ensure remain unique from row to row?

Thanks.

--
Brandon