UPDATE requests
SET patient_id = id_map.new_id
FROM (
    SELECT id AS old_id,
    MIN(id) OVER (PARTITION BY participant_id, external_id) AS new_id
    FROM patients
) AS id_map
WHERE patient_id = id_map.old_id
AND id_map.old_id != id_map.new_id;
  
DELETE FROM patients
   WHERE id NOT IN (
     SELECT MIN(id)
     FROM patients
     GROUP BY participant_id, external_id
   );