mysql> SELECT * from test;
+---+---+
| a | b |
+---+---+
| a | a |
| a | a |
| a | b |
| b | b |
+---+---+
4 rows in set (0.00 sec)

mysql> SELECT * FROM test WHERE a = 'a';
+---+---+
| a | b |
+---+---+
| a | a |
| a | a |
| a | b |
+---+---+
3 rows in set (0.00 sec)

mysql> SELECT DISTINCT * FROM test WHERE a = 'a';
+---+---+
| a | b |
+---+---+
| a | a |
| a | b |
+---+---+
2 rows in set (0.00 sec)