When you need to show values in a SQL table (RDBMS) with an objective to display values which are duplication, and summarize the same value by count it as a numeric. So, you have to write SQL statement and execute it as an example follow;

Table name “ExamResults”

Id	Student	Grade
1	Steven	3.5
2	Dave	3.7
3	Ryan	3.25
4	Jamie	4.0
5	Eric	3.5
6	Robbie	4.0
7	Peter	4.0
8	John	3.25
9	Gary	4
10	Roy	2.85

Above table you will see a student with their grade. So, we need to find “how many students who are get grade 4 in the class?”. This question can be responded with our objective to find duplicated values in a table. You have to design your SQL statement using SELECT & GROUP function as follow;

SELECT field_name, COUNT(field_name) AS icount
FROM TABLE_NAME
GROUP BY field_name

to be

SELECT Grade, COUNT(Grade) AS icount
FROM ExamResults
GROUP BY Grade

Result

Grade	icount
3.5	2
3.7	1
3.25	2
4	4
2.85	1

This results will tell us how many student each of grades. The number 1 in a column of “icount” is mean to no student in a class which get that grade more than one people, unduplication.

Additional,  you can specify to see only a duplicated values in the table by define HAVING function  as follow;

SELECT Grade, COUNT(Grade) AS icount
FROM ExamResults
GROUP BY Grade
HAVING COUNT(Grade) > 1

Result

Grade	icount
3.5	2
3.25	2
4	4

Moreover, if you need to see the duplicated value under  your conditions, you can use WHERE as a condition in your statement as follow;

SELECT Grade, COUNT(Grade) AS icount
FROM ExamResults
WHERE Grade<4
GROUP BY Grade
HAVING COUNT(Grade) > 1

Result

Grade	icount
3.52	2
3.25	2

NOTE: SQL statements in this content are fine to execute in MSSQL, but can be applied to use in the other, i.e. MySQL, PostgresQL etc.