A candidate is resitting for Component 2 - Essay, can they use the same topic with a different essay, or does he have to come up with an entirely new topic?

If the candidate wants to retake component 02, they need to submit new work, not a reworked version of previous submissions. So an essay on the same topic, using the same research and with the same perspectives would not qualify and be rejected. A completely different essay with just the same title would be fine.