SQLite 3.27

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CREATE TABLE tablename (`name` VARCHAR(8), `surname` VARCHAR(3), `nickname` VARCHAR(5));
INSERT INTO tablename (`name`, `surname`, `nickname`) VALUES
  ('Jeniffer', 'Doe', 'Jenny'),
  ('Jeniffer', 'Doe', NULL),
  ('Jeniffer', 'Doe', 'Jenny'),
  ('John', 'Doe', NULL);
DELETE FROM tablename
WHERE rowid NOT IN (
  SELECT COALESCE(
           MIN(CASE WHEN nickname IS NOT NULL THEN rowid END),
           MIN(rowid)
         )  
  FROM tablename 
  GROUP BY name, surname
);
SELECT * FROM tablename;


name
surname
nickname


Jeniffer
Doe
Jenny

John
Doe
null