Fix for bug#5400 "GROUP_CONCAT returns everything twice":

Don't evaluate the value of GROUP_CONCAT several times for the same 'group', reuse the value instead.

mysql-test/r/group_by.result

@@ -629,3 +629,12 @@ explain SELECT i, COUNT(DISTINCT(i)) FROM t1 GROUP BY j ORDER BY NULL;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	6	Using filesort
 DROP TABLE t1;
+create table t1 ( col1 int, col2 int );
+insert into t1 values (1,1),(1,2),(1,3),(2,1),(2,2);
+select group_concat( distinct col1 ) as alias from t1
+group by col2 having alias like '%';
+alias
+1,2
+1,2
+1
+drop table t1;

mysql-test/t/group_by.test

@@ -456,3 +456,12 @@ INSERT INTO t1 VALUES (1,2),(2,3),(4,5),(3,5),(1,5),(23,5);
 SELECT i, COUNT(DISTINCT(i)) FROM t1 GROUP BY j ORDER BY NULL;
 explain SELECT i, COUNT(DISTINCT(i)) FROM t1 GROUP BY j ORDER BY NULL;
 DROP TABLE t1;
+
+# Test for BUG#5400: GROUP_CONCAT returns everything twice.
+create table t1 ( col1 int, col2 int );
+insert into t1 values (1,1),(1,2),(1,3),(2,1),(2,2);
+select group_concat( distinct col1 ) as alias from t1
+  group by col2 having alias like '%';
+
+drop table t1;
+

sql/item_sum.cc

@@ -2121,6 +2121,8 @@ String* Item_func_group_concat::val_str(String* str)
   DBUG_ASSERT(fixed == 1);
   if (null_value)
     return 0;
+  if (result.length())
+    return &result;
   if (tree_mode)
   {
     tree_walk(tree, (tree_walk_action)&dump_leaf_key, (void*)this,