Odd double queries continues

My odd "double" queries continue. On the theory that I had some kind of
page-faulting issue tied into large, frequent updates of a table, I vacuumed
the whole database and began to watch it closely this morning. Already,
after very little activity, I get this in the log:

2004-08-26 07:01:26 [22056] DEBUG: query: INSERT INTO nsse_session
(id_user, id_session, expiration, value) VALUES (<values>)
2004-08-26 07:01:26 [22056] ERROR: Cannot insert a duplicate key into
unique index nsse_session_pkey
2004-08-26 07:01:26 [22056] DEBUG: query: UPDATE nsse_session SET <values>
WHERE id_session = '32845b87350846bf51176079c19f2fb4'
EXECUTOR STATISTICS
! system usage stats:
! 0.000183 elapsed 0.000000 user 0.000000 system sec
! [0.000000 user 0.030000 sys total]
! 0/0 [0/0] filesystem blocks in/out
! 2/4 [414/257] page faults/reclaims, 0 [0] swaps
! 0 [0] signals rcvd, 0/0 [0/0] messages rcvd/sent
! 0/0 [0/0] voluntary/involuntary context switches
! postgres usage stats:
! Shared blocks: 0 read, 0 written, buffer hit rate
= 100.00%
! Local blocks: 0 read, 0 written, buffer hit rate
= 0.00%
! Direct blocks: 0 read, 0 written
EXECUTOR STATISTICS
! system usage stats:
! 10.626398 elapsed 10.490000 user 0.010000 system sec
! [10.510000 user 0.010000 sys total]
! 0/0 [0/0] filesystem blocks in/out
! 261/10 [691/281] page faults/reclaims, 0 [0] swaps
! 0 [0] signals rcvd, 0/0 [0/0] messages rcvd/sent
! 0/0 [0/0] voluntary/involuntary context switches
! postgres usage stats:
! Shared blocks: 0 read, 0 written, buffer hit rate
= 100.00%
! Local blocks: 0 read, 0 written, buffer hit rate
= 0.00%
! Direct blocks: 0 read, 0 written

Same pattern as before -- the same query seems to get run twice, the second
time ridiculously long, with a high number of page faults. (Again, this is a
straightforward query involving a single index scan, and the planner says as
much).

I realize no one may have any insight in the whole problem, but can anyone
at least tell me the significance of each of these numbers from the stats:

261/10 [691/281] page faults/reclaims, 0 [0] swaps

That might give me an idea of where to begin.

-- sgl