What is your guidance and instruction regarding repentance for a sin committed by the questioner after having reproached another person for the same sin, and is he required to confess to that person and ask for their forgiveness?

The conditions for repentance are: regret for the sin, desisting from it, and resolving not to return to it.

For transgressions involving the rights of others—such as backbiting—an additional condition is seeking absolution from the wronged person. If the person is unaware of it, then seek their forgiveness without detailing the specifics. If informing them would lead to greater harm, then suffice with supplicating for them, seeking forgiveness for them, and speaking well of them.

Repentance for a specific sin, to the exclusion of others, is valid according to the most preponderant opinion, contrary to what might be understood from some statements. Indeed, the validity of repenting for some sins is a fundamental principle of the Sunnah.