limited field duplicates

[Don Isgitt <djisgitt(at)soundenergy(dot)com>]

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