SQL Server 2017

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CREATE TABLE [User] (
  user_id int NOT NULL,
  PAios_AccountId varchar(50) NOT NULL,
);
INSERT INTO [User] (user_id, PAios_AccountId) VALUES
  (123, 'a'),
  (223, 'b'),
  (123, 'b'),
  (223, 'c'),
  (123, 'd');
5 rows affected

WITH cte AS (
SELECT *, ROW_NUMBER() OVER (PARTITION BY user_id ORDER BY PAios_AccountId) AS rn 
FROM [User]
)
DELETE FROM cte
WHERE rn <> 1
3 rows affected

SELECT *
FROM [User]


user_id
PAios_AccountId


123
a

223
b