Duplicate Key -- Have I missed something

I have a production environment that is running the code that
caused this problem about once every 30 seconds or so, with two
clients. It normally runs very well, but I was suddenly hit with a
duplicate key violation for a table where the primary key is
generated in part from sequence.

How might this have happen?

ERROR: duplicate key violates unique constraint
"game_pay_cash_coupon_trans_pkey"
CONTEXT: SQL statement "INSERT INTO game_pay_cash_coupon_trans
( ttype, lid, id, gpid, cash_paid, trans_user ) values( 'INSERT',
$1 , $2 , $3 , $4 , $5 )"
PL/pgSQL function "game_pay_cash_coupon_add" line 7 at SQL
statement
SQL statement "SELECT game_pay_cash_coupon_add( $1 , gpk,
cash_in_use ) FROM game_state_game gsg, game_state_cash_coupon gscc
WHERE gsg.gid = $2 AND gscc.gid = $3 "
PL/pgSQL function "game_state_complete_payment" line 13 at
perform
SQL statement "SELECT game_state_complete_payment( $1 ,
$2 )"
PL/pgSQL function "game_state_game_complete" line 20 at perform

The offeded table

Table
"public.game_pay_cash_coupon_trans"
Column | Type
| Modifiers
------------+-----------------------------------
+------------------------------------------------------------------
toid | d_location_id | not null default
"location"()
tid | d_trans_id | not null default
nextval('game_pay_cash_coupon_trans_seq'::text)
ttype | d_game_pay_cash_coupon_trans_type |
lid | d_location_id |
id | d_game_pay_cash_coupon_id |
gpid | d_game_play_id |
cash_paid | d_money |
trans_user | d_user_name |
trans_time | d_timestamp |
Indexes:
"game_pay_cash_coupon_trans_pkey" PRIMARY KEY, btree (toid, tid)
Triggers:
trig_game_pay_cash_coupon_trans_aft_ins AFTER INSERT ON
game_pay_cash_coupon_trans FOR EACH ROW EXECUTE PROCEDURE
trig_func_game_pay_cash_coupon_trans_aft_ins()
Has OIDs: no

The sequence in question

mallball=> select * from game_pay_cash_coupon_trans_seq;
sequence_name | last_value | increment_by |
max_value | min_value | cache_value | log_cnt | is_cycled |
is_called
--------------------------------+------------+--------------
+---------------------+-----------+-------------+---------+-----------
+-----------
game_pay_cash_coupon_trans_seq | 100000815 | 1 |
9223372036854775807 | 1 | 1 | 32 | f | t

The build in question

mallball=> select * from version();
version
------------------------------------------------------------------------
-------------------------------
PostgreSQL 8.0.1 on i686-pc-linux-gnu, compiled by GCC gcc (GCC)
3.4.2 20041017 (Red Hat 3.4.2-6.fc3)

Thanks
Robert Perry