Duplicate values can be found using this
SELECT email, COUNT(email)
FROM users
GROUP BY email
HAVING COUNT(email) > 1
for the data
ID NAME EMAIL
1 John asd@example.com
2 Sam asd@example.com
3 Tom asd@example.com
4 Bob bob@example.com
5 mob bob@example.com
But the table I need is
NAME EMAIL
John, Sam, Tom asd@example.com
Bob, mob bob@example.com