Deleting and inserting with unique index

I'm getting a database error when deleting and inserting records in a single unit of work. Consider this example:

I have a user and user_info tables. user_info stores additional data about a user as a list of key-values.
user: id, name, email, password
user_info: id, user_id, info_key, info_value

I created a unique index on user_info table: CREATE UNIQUE INDEX `user_info_key` ON `user_info` (`user_id` , `key` ) ;

Each table is represented as an Entity class.

Now, I run this code:
1. $user->removeAllInfo(); // remove old entries
2. $user->addInfo('first_name', 'John'); // add new entries
3. $this->em->flush(); // save

This causes MySQL error: "Integrity constraint violation: 1062 Duplicate entry". Doctrine ORM first persists new entities and then deletes old entities. I think the relevant code is at Doctrine/ORM/UnitOfWork :277-313. Everything works fine if I add flush() before line 2. Also, everything works fine if I change the index to non-unique.