UNION makes strange duplicates

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$psql -V
psql (PostgreSQL) 7.4.7
contains support for command-line editing
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I have table with two _id (document_id and document_store_id). And I
have a view:

SELECT document_items.document_store_id AS document_id,
document_items.vat_type_id, vat_type_value,
sum(document_items.document_item_value) AS document_netto_value
FROM document_items
JOIN vat_type ON document_items.vat_type_id = vat_type.vat_type_id
GROUP BY document_items.document_store_id,
document_items.vat_type_id, vat_type_value
UNION
SELECT document_items.document_id, document_items.vat_type_id,
vat_type_value, sum(document_items.document_item_value) AS
document_netto_value
FROM document_items
JOIN vat_type ON document_items.vat_type_id = vat_type.vat_type_id
GROUP BY document_items.document_id, document_items.vat_type_id,
vat_type_value;

each of these two SELECTs almost always produce the same results
(because document_store_id almost always is equal to document_id , so
the result of VIEW should be exact the same as result of each SELECT.

BUT it isn't - example:
(SELECT * FROM document_values_by_vat WHERE document_id = '65615')

document_id vat_type_id vat_type_value document_netto_value
65615 1 0 0
65615 4 0.07 -12.5327
65615 5 0.22 -7.31148
65615 5 0.22 -7.31148

the expected result is:
document_id vat_type_id vat_type_value document_netto_value
65615 1 0 0
65615 4 0.07 -12.5327
65615 5 0.22 -7.31148

the result of first select is:
document_id vat_type_id vat_type_value document_netto_value
65615 1 0 0
65615 4 0.07 -12.5327
65615 5 0.22 -7.31148

the result of second select is:
document_id vat_type_id vat_type_value document_netto_value
65615 1 0 0
65615 4 0.07 -12.5327
65615 5 0.22 -7.31148

Any ideas?
--
T.