Laravel Testing: Can not login 2 users in a same test case with JWT

My application use JWT to handle authentication. In a testcase, I try to login with 2 users, get the tokens to call other APIs. But seem like it act as 1 users (the first one)

Check out this code, after login, I call API to get user information. But it always return the information of the user 1.

private function authenticate()
{
    $user = factory(User::class)->create();
    $token = \JWTAuth::fromUser($user);
    return [
        'Authorization' => 'Bearer ' . $token,
    ];
}

public function test2Login()
{
    $user1 = $this->authenticate();
    $user2 = $this->authenticate();
    // This to make sure the token are different
    $this->assertNotEquals($user1['Authorization'], $user2['Authorization']);

    // Call API to get user information by the token
    $response1 = $this->withHeaders($user1)->get('api/user');
    $response2 = $this->withHeaders($user2)->get('api/user');

    $userId1 = $response1->json()['data']['id'];
    $userId2 = $response2->json()['data']['id'];
    // Both response return information of the same user, so this testcase fail
    $this->assertNotEquals($userId1, $userId2);
}

I expect $userId1 and $userId2 are different. For now it return the same id.

I debug into GetUserFromToken.php, which is the Middleware of JWT. The tokens it received in both request are the same. This is weird cause in the testcase I did make sure the authenticate function return 2 different tokens.

Anyone have any idea? Thanks