PDOException: SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry '2-4' for key 1: INSERT INTO {users_roles}

I'm trying to create a rule to block a new user with a particular role, after the rule is added, when admin tries to add a new user with this role, I got the following exception:

PDOException: SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry '2-4' for key 1: INSERT INTO {users_roles} (uid, rid) VALUES (:db_insert_placeholder_0, :db_insert_placeholder_1); Array ( [:db_insert_placeholder_0] => 2 [:db_insert_placeholder_1] => 4 ) in user_save() (line 595 of drupal-7.12\modules\user\user.module).

Setup: Newly installed D7.12 + Rules 7.x-2.0+16-dev + Entity API 7.x-1.0-rc1+34-dev with no other modules. Add new role "role1", add new rule:

{ "rules_block_role1_login" : {
    "LABEL" : "Block role1 login",
    "PLUGIN" : "reaction rule",
    "REQUIRES" : [ "rules" ],
    "ON" : [ "user_insert" ],
    "IF" : [
      { "user_has_role" : { "account" : [ "account" ], "roles" : { "value" : { "4" : "4" } } } }
    ],
    "DO" : [ { "user_block" : { "account" : [ "account" ] } } ]
  }
}