User gets registered to the challenge again after they unregister

[SathyaJayabal]

1. Go to Challenge details page
2. Register to the challenge (example: https://www.topcoder.com/challenges/08b74bb4-67df-49a4-bd8e-672ac5e257d2)
3. Unregister to the challenge
4. After a couple of minutes, user is registered back to the challenge, and even receives Registration successful email again.

Expected: user should not be registered to the challenge again and should stay unregistered.

cc @Oanh-and-only-Oanh