What is the ruling on one who repented from backbiting, then backbit, unintentionally and forgetfully, so he sought forgiveness—is he required to renew his repentance?

If someone relapses into a sin after repentance, they must renew their repentance. If they relapse into backbiting, they must renew their repentance. Claiming it happened unintentionally is unacceptable, because a person must be cautious in their speech, for most of a human being's sins are due to their tongue.