DISTINCT ON not working...?

Hi all,

Strange one - I have a nightly export / import routine that exports from one
database and imports to another. Has been working fine for several months,
but last night it died on a unique constraint.

To cut out all the details, the code that is causing the problem:
SELECT DISTINCT ON (ean)
code,
CASE WHEN ean IS NULL OR valid_barcode(ean) = false THEN
null ELSE ean END AS ean
FROM TMPTABLE
WHERE code NOT IN (SELECT code FROM stock_deleted)
AND ean IS NOT NULL

That is the code that generates the error on the unique constraint against
the ean column.

If I play with that and run this:
SELECT DISTINCT ON (ean)
CASE WHEN ean IS NULL OR valid_barcode(ean) = false THEN
null ELSE ean END AS ean,
count(*)
FROM TMPTABLE
WHERE code NOT IN (SELECT code FROM stock_deleted)
AND ean IS NOT NULL
GROUP BY ean

I get a several thousand rows returned, all with a count(*) of 1, except one
row:
3246576919422 2

DISTINCT ON should eliminate one of those rows that is making that 2 - as I
said, it's been working fine for several months, and it is still doing it
correctly for approximately 100 other rows that have duplicate ean codes.

Can anyone give me a hand to work out why this one is doubling up?!

Cheers,
~p

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