Urgent: Key constraints behaving weirdly

Your name : Russ Garrett
Your email address : russ(at)last(dot)fm

System Configuration
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Architecture (example: Intel Pentium) : P4 Xeon
Operating System (example: Linux 2.0.26 ELF) : Linux 2.4.22 ELF
PostgreSQL version (example: PostgreSQL-7.4) : PostgreSQL-7.4
Compiler used (example: gcc 2.95.2) : gcc version 3.2.2 20030222
(Red Hat Linux 3.2.2-5)

Please enter a FULL description of your problem:
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Constraints are being weird. The reproduction instructions speak for
themselves.
We're using this on a production database and we're understandably getting a
bit edgy.

Please describe a way to repeat the problem. Please try to provide a
concise reproducible example, if at all possible:
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last=> \d profile
...
Indexes:
"profile_pkey" primary key, btree (id)

last=> \di+ profile_pkey
List of relations
Schema | Name | Type | Owner | Description | Table
--------+--------------+-------+--------+-------------+---------
public | profile_pkey | index | lastfm | | profile
(1 row)

last=> UPDATE profile SET lastsongtime=now(), r1=r1+0, r2=r2+0, r3=r3+0,
r4=r4+0, r5=r5+0, scrobcount=scrobcount+1 WHERE id=1017057;
ERROR: duplicate key violates unique constraint "profile_pkey"

last=> SELECT * FROM profile WHERE id=1017057;
id | user | name | description | songspaminterval |
lastsongtime | entropy | newbie | creationdate | r1 | r2 |
r3 | r4 | r5 | scrobcount | newbielistenas | e1 | e2 | e3 | e4 | e5 |
numdisconnections | totalstreamtime
---------+---------+---------+-------------+------------------+-------------
--------+---------+--------+----------------------------+----+----+----+----
+----+------------+----------------+----+----+----+-----+----+--------------
-----+-----------------
1017057 | 1017075 | Default | | 60 | 2003-12-18
04:32:34 | 1 | f | 2003-12-17 23:57:01.582757 | 0 | 0 | 0 | 0
| 0 | 23 | | 2 | 0 | 0 | 219 | 0 |
0 | 0
(1 row)

last=>

If you know how this problem might be fixed, list the solution below:
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No idea :).