CREATE TABLE Students (
    Id INT PRIMARY KEY,
    Name VARCHAR(50),
    Age INT
);

INSERT INTO Students VALUES (1, 'John', 20);
INSERT INTO Students VALUES (2, 'Mary', 22);
INSERT INTO Students VALUES (3, 'John', 20);
INSERT INTO Students VALUES (4, 'Mary', 22);
INSERT INTO Students VALUES (5, 'John', 18);

2, 'Mary', 22
4, 'Mary', 22

SELECT Name, Age, COUNT(*) AS Count
FROM Students
GROUP BY Name, Age
HAVING COUNT(*) > 1;

Name  |  Age  |  Count
---------------------
John  |  20   |  2
Mary  |  22   |  2

SELECT Id, Name, Age, COUNT(*) OVER(PARTITION BY Name, Age) AS Count
FROM Students
WHERE COUNT(*) OVER(PARTITION BY Name, Age) > 1;

Id  |  Name  |  Age  |  Count
----------------------------
1   |  John  |  20   |  2
3   |  John  |  20   |  2
2   |  Mary  |  22   |  2
4   |  Mary  |  22   |  2

DELETE FROM Students
WHERE Id NOT IN (
    SELECT MIN(Id)
    FROM Students
    GROUP BY Name, Age
);