I heard a theory that this was no clone...

The real Sephiroth was killed by cloud(as everybody knows) but because of the will too find Jenova he refused too become a part of the lifestream..
Soo the shirna inc transported his damaged body to Northen crater and placed it in a mako chrysalis.

There with he´s great willpower he was so strong that he could coalesce himself into physical form through is mind and travel that way.

So you could say that Sephiroth was the killer

But... as i say just a theory that is not to be taking seriously.