Bug #3381 Strange behaviour with GROUP_CONCAT() using DISTINCT

Description:
GROUP_CONCAT(DISTINCT ...) fails, the result is non-distinct. I originally thought there 
might be a connection with BUG##2695  Strange behaviour with GROUP_CONCAT() using 
ORDER BY. But we will now consider it a separate bug. 
 
 

How to repeat:
 
mysql> create table t2 (s1 char(10));  
Query OK, 0 rows affected (0.04 sec)  
  
mysql> insert into t2 values ('a'),('b'),('c'),('a');  
Query OK, 4 rows affected (0.00 sec)  
Records: 4  Duplicates: 0  Warnings: 0  
  
mysql> select group_concat(distinct s1) from t2;  
+---------------------------+  
| group_concat(distinct s1) |  
+---------------------------+  
| a,c,b,a                   |  
+---------------------------+  
1 row in set (0.00 sec)

Thank you for your bug report. This issue has been committed to our
source repository of that product and will be incorporated into the
next release.

If necessary, you can access the source repository and build the latest
available version, including the bugfix, yourself. More information 
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Additional info:

Fix will be in 4.1.2
See #2695 for details