proper DAO way to do a 'save if not in the db'?

Question

Here's what I have, which has problems:

public User addUser(final String name, final String uid) {
    final List<User> result = findByProperty("uid", uid);

    if (!result.isEmpty())
        return (result.get(0));

    final User user = new User(name, uid);

    saveOrUpdate(user);

    return (user);
}

where findByProperty is using a Criteria and adding

Restrictions.eq(propertyName, propertyValue))

The uid in this case is not the table's primary key, it's from another system (ldap), although it is unique.

The main problem that I see with my code, and there may be others, is that there's window of opportunity for a collision, between the check for existence and the saveOrUpdate.

Thanks for your help.