Comment 1 for bug 314570

Confirmed, but only for the InnoDB storage engine, which should help narrow down the fix:

jpipes@serialcoder:~/repos/drizzle/new-temporal/tests$ ../client/drizzle --user=root --port=9306
Welcome to the Drizzle client.. Commands end with ; or \g.
Your Drizzle connection id is 1
Server version: 7.0.0 Source distribution

Type 'help;' or '\h' for help. Type '\c' to clear the buffer.

drizzle> use test
Database changed
drizzle> create table t1 (a int not null auto_increment primary key, val int)engine=innodb;
Query OK, 0 rows affected (0.01 sec)

drizzle> insert into t1 (val) values (1);
Query OK, 1 row affected (0.00 sec)

drizzle> update t1 set a=2 where a=1;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0

drizzle> insert into t1 (val) values (1);
ERROR 1062 (23000): Duplicate entry '2' for key 'PRIMARY'
drizzle> drop table t1;
Query OK, 0 rows affected (0.01 sec)

drizzle> create table t1 (a int not null auto_increment primary key, val int)engine=myisam;
Query OK, 0 rows affected (0.02 sec)

drizzle> insert into t1 (val) values (1);
Query OK, 1 row affected (0.00 sec)

drizzle> update t1 set a=2 where a=1;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0

drizzle> insert into t1 (val) values (1);
Query OK, 1 row affected (0.00 sec)