Zack technically became a failed clone of Sephiroth. Doctor Hojo injected Zack with the same Jenova Cells as Sephiroth; however, it did not have much of an effect on him. We can also consider a Sephiroth clone since Doctor Hojo injected Cloud with the Jenova Cells. In the world of Final Fantasy VII, anyone injected with cells of a superior being can be called a clone.