SQL Server 2019

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CREATE TABLE #names (
  first_name VARCHAR(100),
  last_name VARCHAR(100)
);
INSERT INTO #names VALUES
('Moe', 'Momo'),
('Moe', 'Momo'),
('Jack', 'JAJA'),
('Jack', 'Jacky');
4 rows affected

SELECT
  n.first_name,
  CASE WHEN MIN(n.last_name) = MAX(n.last_name) THEN MIN(n.last_name) END AS last_name
FROM #names n
GROUP BY
  n.first_name;


first_name
last_name


Jack
null

Moe
Momo