mysql> USE test;
DATABASE changed
mysql> CREATE TABLE mirko_test (foo INT, bar INT);
Query OK, 0 ROWS affected (0.61 sec)

mysql> INSERT INTO mirko_test VALUES (5, 1), (5, 1), (5, 1), (10, 1), (5, 2), (10, 2);
Query OK, 6 ROWS affected (0.00 sec)
Records: 6  Duplicates: 0  Warnings: 0

mysql> SELECT * FROM mirko_test;
+------+------+
| foo  | bar  |
+------+------+
|    5 |    1 |
|    5 |    1 |
|    5 |    1 |
|   10 |    1 |
|    5 |    2 |
|   10 |    2 |
+------+------+
6 ROWS IN SET (0.00 sec)

mysql> SELECT avg(foo), bar FROM mirko_test GROUP BY bar;
+----------+------+
| avg(foo) | bar  |
+----------+------+
|   6.2500 |    1 |
|   7.5000 |    2 |
+----------+------+
2 ROWS IN SET (0.00 sec)

mysql> SELECT avg(foo), bar FROM (SELECT DISTINCT * FROM mirko_test) t GROUP BY bar;
+----------+------+
| avg(foo) | bar  |
+----------+------+
|   7.5000 |    1 |
|   7.5000 |    2 |
+----------+------+
2 ROWS IN SET (0.00 sec)

mysql>