MySQL 8.0

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CREATE TABLE Computer(
  ComputerID int,
  ComputerCN VARCHAR(20) );
  


INSERT INTO Computer VALUES
(3,'testmachine'),
(5,'testmachine'),
(6,'testmachine11111');

Records: 3  Duplicates: 0  Warnings: 0


SELECT * from Computer;


ComputerID
ComputerCN


3
testmachine

5
testmachine

6
testmachine11111


SELECT ComputerID 
FROM Computer 
WHERE ComputerCN IN ( SELECT ComputerCN
                      FROM Computer
                      GROUP BY ComputerCN
                      HAVING COUNT(*) > 1
                     );


ComputerID


3

5


WITH duplicates AS (
    SELECT *, 
           COUNT(*) OVER (PARTITION BY ComputerCN) dupes
    FROM Computer
)
SELECT ComputerID
FROM duplicates
WHERE dupes > 1


ComputerID


3

5