mysql> create table dupes (id int, txt char);
Query OK, 0 rows affected (0.09 sec)

mysql> insert into dupes values (1,"a"),(2,"b"),(3,"c"),(4,"a"),(5,"b");
Query OK, 5 rows affected (0.02 sec)
Records: 5  Duplicates: 0  Warnings: 0

mysql> select * from dupes;
+------+------+
| id   | txt  |
+------+------+
|    1 | a    |
|    2 | b    |
|    3 | c    |
|    4 | a    |
|    5 | b    |
+------+------+
5 rows in set (0.01 sec)

mysql> select txt, count(*) from dupes group by txt having count(*) > 1;
+------+----------+
| txt  | count(*) |
+------+----------+
| a    |        2 |
| b    |        2 |
+------+----------+
2 rows in set (0.07 sec)