Except all eliminates what it shouldnt

Gerzson Szinyei (gerzson@elender.hu) reports a bug with a severity of 1
The lower the number the more severe it is.

Short Description
Except all eliminates what it shouldnt

Long Description
"except all" eliminates records which had 2 instances in the original table, and one instance in the table to be
eliminated.One instance should have showed up in the result. 
When I filter the original table with "where field=nnnn", (the id of the problem record), and do the "except all" for
onlythe records in questtion, the result is one instance as it should be. 



Sample Code
http://www.webit.hu/~gerzson/pgexceptbugtest.tgz

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