select where not exists returning multiple rows?

We're using postgresql 9.1, and we've got a table that looks like this:

testdb=# \d item
Table "public.item"
Column | Type | Modifiers
-------+----------+-----------
sig | bigint | not null
type | smallint |
data | text |
Indexes:
"item_pkey" PRIMARY KEY, btree (sig)

And we're doing an insert like this:
INSERT INTO Item (Sig, Type, Data) SELECT $1,$2,$3 WHERE NOT EXISTS (
SELECT NULL FROM Item WHERE Sig=$4)

In this case $1 and $4 should always be the same. The idea is to insert
if the row doesn't already exist.
We're getting primary key constraint violations:

011-10-31 22:50:26 CDT STATEMENT: INSERT INTO Item (Sig, Type, Data)
SELECT $1,$2,$3 WHERE NOT EXISTS ( SELECT NULL FROM Item WHERE Sig=$4
FOR UPDATE)
2011-10-31 22:52:56 CDT ERROR: duplicate key value violates unique
constraint "item_pkey"
2011-10-31 22:52:56 CDT DETAIL: Key (sig)=(-4668668895560071572)
already exists.

I don't see how it's possible to get duplicate rows here, unless maybe
the "select where not exists" is somehow returning multiple rows.
Any ideas what's going on here?

Thanks,
Chris