I hope you're ready for a pretty long post.
We all know that in the beginning of the game, Cloud has confused himself with that of another person. In his story when they were in Kalm, he had taken over the role of Zack. And then we were made to think that Cloud was actually just a Sephiroth clone, until the lifestream event, where Tifa had proven that somewhere in that confused Cloud is the real Cloud, and had brought him out into the open once more.
First we try to explain the Sephiroth Clone = Cloud bit:
If you visited the basement at the Nibelheim mansion after the lifestream event, you'll be able to see a long cut-scene about what happened to both Zack and Cloud after the Nibelheim incident. In an attempt to create a new Sephiroth, Hojo tried to inject liberal amounts of Jenova cells into both Zack and Cloud (and all other survivors) as an experiment. "Clone" at this particular point shouldn't be taken in the literal sense - it just meant that Hojo intenteded to create someone as strong and effective as Sephiroth had been. And since real SOLDIERS are Shinra troops who can withstand extreme effects of Jenova cells being inserted into their bodies, Zack was able to remain sane, whereas Cloud wasn't.
I had to mention that because it's important for my next discussion.
Now we move over to the Cloud = Zack bit, which is where the debate comes in:
The debate had first begun when someone had mentioned that Cloud only fell in love with Aerith because he had taken over Zack's personality and all that. Aerith fans denied this of course, and a few state that Cloud did not confuse all of his memories with Zack at all. A very good debater (the one who had defeated me.

) believed that it was actually Sephiroth who had distorted Cloud's memories as such, all so that he would be even more confused and break his will. Either that, or Cloud was trying to, and I quote, "remove himself from the entire trauma by remembering the incident through the eyes of someone else".
Several proofs had been stated which would prove that Cloud couldn't have had confused his memories with Zack - he knows he's born in Nibelheim instead of Gongaga. He had the memory of talking to his mother. Later in the game, he agreed with Tifa that the people occupying the newly built Nibelheim were different. Zack wouldn't have gone up to Tifa's room to look into her closet, play her piano, and read her letter - that would have been an invasion of privacy, and in someone's house he didn't know, to boot.
Then again... why would Cloud go into Tifa's room?
Since Cloud is trying to avoid Tifa during the entire Nibelheim incident, why in the world would he brave going into her house, look into her closet, play her piano, and memorize her letter, knowing that in any minute Tifa or her father could come in and see him?
Going back to the game, we hear Sephiroth say the following:
QUOTE
Sephiroth: Cloud... Don't blame Tifa. The ability to change one's looks, voice, and words, is the power of Jenova. Inside of you, Jenova has merged with Tifa's memories, creating you. Out of Tifa's memory...... A boy named Cloud might've just been a part of them.
So the Jenova cells had merged with Tifa's memories. That explained my question earlier, right?

And since it happened to Cloud with Tifa, why not with Zack's?
So my argument is that it is still possible that Cloud had confused his memories with that of both Zack
and Tifa, thanks to those Jenova cells merging with their memories, mixing and adding them up along with Cloud's other memories in his head. And since the Jenova cells are more powerful than his own, Cloud's own memories of his own role during the incident gets pushed back in favor of these new ones.
Now let's return to the proofs that Cloud had not confused his memories with Zack:
- Cloud knows he's born in Nibelheim instead of Gongaga.
Tifa would know this.
- He had the memory of talking to his mother.
He did. But compared to all the other recollections he had, this one had been pretty vague.
- Cloud agreed with Tifa that the people occupying the newly built Nibelheim were different.
Tifa would know the townspeople. Even more so than Cloud, considering that he had been away for two years until the incident occurred.
- Zack wouldn't have gone up to Tifa's room.
Tifa would know what's in her own closet, would know what's written in a letter addressed to her.
Just several additional ones my main opponent pointed out:
- Cloud recognized Tifa before she recognized him at the train station
Zack would know who Tifa was during the Nibelheim incident.
- He recognized Jenova in Hojo's lab
Zack would know Jenova, too.
- He recognizes President Shinra
So would Zack.
However, as with all theories, mine had several flaws which my opponent had pointed out:
- If Tifa's memories got mixed up with Cloud's, how come he couldn't remember the promise they made until Tifa reminded him?
- Sephiroth mentioned, in the same scene where he said Tifa's memories had merged with Cloud's, that Cloud had been constructed by Hojo, piece by piece. Which, in the end, we know is a lie. So does this mean that the one he said about memories was a lie as well?
The part about the promise is where I knew that I had lost. I couldn't explain why Cloud couldn't remember the promise, especially when it had been an important memory for Tifa.
Even though Tifa had said the following:
QUOTE
Tifa: ... You told me a lot about what happened after you left Nibelheim...
But... Something's wrong. I felt there was something strange about the things
you talked about. All the things you didn't know that you should. And other
things you shouldn't know that you did...
... and even though I could probably say that the things Cloud knows that he shouldn't doesn't only have to be the Nibelheim incident (since she didn't know he was there), and the things Cloud should know but doesn't doesn't have to be limited to the promise, I don't have evidence to prove this. So, I lost gracefully, declared the winner, and bowed out of the debate.
Anyway, that's the entire discussion on the Zack = Cloud thing. Choose which you want to believe.

I admit, I lost the debate, but I still believe in my theory, and am trying to find further evidence to start this discussion over at that thread again.
If you've made it this far, thanks for reading! Hehe...