count (*)

alonso wrote:

I've got it, but I don't know the reason of it.
In the database table the records have duplicated and I've
two rows for
every primary key - with different OIDs.
Very strange however...

Very strange indeed.

Are you positive that there is a primary key constraint on
the column? Can you show some evidence?

Is there any safe method to get rid of only one of each
duplicated row?

If "id" is your "primary key":

DELETE FROM test USING test a
WHERE test.id = a.id AND test.oid > a.oid;

Yours,
Laurenz Albe