What I understood is that Kairi was able to bring Sora back with pure power of will and strength of her own heart. Sora is still the original Sora. It's like when someone revives a dead person in fiction, the revived person is not a clone, they are themselves.is Sora actually a clone of himself since Roxas is the original body? Sora's heart became a heartless then that heart reconstructed a body since his Nobody/original body wasn't destroyed by a keyblade?
Not exactly. What happened was that back when Destiny Island was destroyed, Kairi's heart got attached to Sora. When Sora became a Heartless, Roxas was born as Sora's Nobody and Naminé was born as Kairi's Nobody (since Kairi's heart was inside Sora's body at the time), but because of the special condition of Kairi's heart being with Sora, Naminé had the powers to manipulate Sora's memories.