From PostgreSQL wiki
A frequent question in IRC is how to delete rows that are duplicates over a set of columns, keeping only the one with the lowest ID.
This query does that for all rows of tablename having the same column1, column2, and column3.
DELETE FROM tablename WHERE id IN (SELECT id FROM (SELECT id, row_number() over (partition BY column1, column2, column3 ORDER BY id) AS rnum FROM tablename) t WHERE t.rnum > 1);
Sometimes a timestamp field is used instead of an ID field.