limited field duplicates

Hello,

I am trying to find rows in a table that are duplicates only in the
sense of certain columns; i.e, not all columns are the same, and then
for each duplicate entry, I want to retrieve one of the columns that is
not part of the duplication criteria. Ok, enough English description,
here is the real information (simplified).
                Table "master" --~925000 records
      Column       |     Type     | Modifiers
-------------------+--------------+-----------
 apinum            | text         | not null
 state             | character(2) | not null
 county            | text         | not null
 township_n        | integer      |
 township_d        | character(1) |
 range_n           | integer      |
 range_d           | character(1) |
 section           | integer      |
 location          | text         | not null

(Indexes on state,county,township,range,apinum)

Find duplicates based on
state,county,township_n||township_d,range_n||range_d,section

select state,county,township_n||township_d as township,range_n||range_d
as range,section,location,count(*) from master where state='OK' and
county='MAJOR' group by state,county,township,range,section,location
having count(*)>1;

Works great, is efficient: now, how do I get the apinum associated with
each of the limited duplicate rows (i.e., those returned in the above
query). I need the apinum because another system involved only knows
about apinum.

It must be simple, but I have failed miserably in finding the answer.
The following horrible query works, but it takes forever for even the
small sample above.

 explain select m.apinum,m.township_n||m.township_d as
township,m.range_n||m.range_d as range,m.section,m.location from master
m where (select count(*) from master m1 where
m.township_n||m.township_d=m1.township_n||m1.township_d and
m.range_n||m.range_d=m1.range_n||m1.range_d and m.section=m1.section and
m.location=m1.location and m1.state='OK' and m1.county='MAJOR') > 1 and
m.state='OK' and m.county='MAJOR' order by township,range,section;
NOTICE:  QUERY PLAN:

Sort  (cost=7040332.16..7040332.16 rows=450 width=55)
  ->  Index Scan using mstcty on master m  (cost=0.00..7040312.32
rows=450 width=55)
        SubPlan
          ->  Aggregate  (cost=5208.86..5208.86 rows=1 width=0)
                ->  Index Scan using mstcty on master m1
(cost=0.00..5208.85 rows=1 width=0)

EXPLAIN


So, somebody please enlighten me. All help is appreciated.

Don