Is Darth Sidious really Darth Sidious?

Question

In The Rise of Skywalker we learn of one of Darth Sidious' plans:

To have Rey strike him down as part of a Sith "ritual", allowing him to in some way possess her body and live on in her.

Looking back this seems very relevant to a Sith history filled with apprentices who have a habit of murdering their masters. So while it is possible that Darth Sidious is the first to really figure this out, it seems more likely (to me anyway) that in the distant past some Sith Lord figured this out and has mastered the art of using the inevitable treachery of an apprentice to effectively attain immortality. If that were the case then it seems quite likely that Sidious was in some way possessed when he killed his own master shortly after becoming Emperor (and therefore he is no longer "himself").

In essence, is it possible that Sidious in the latter stories is simply the latest host body for an ancient Sith Lord who has mastered body hopping?

I suppose it's also possible that this technique has been around for a long time but is rarely used because it is difficult to succeed at (example: Sidious failed to perform the ritual himself). In this case Sidious' attempt may be viewed as a last-ditch and ultimately futile attempt to start over. Still, it seems that if there was a ritual allowing a Sith Lord to take over the body of another, someone in the distant past would have had the time necessary to perfect this art.

If all of Sith history could be seen as a matchup between "Apprentice who wants to kill his master" and "Master who has had dozens of past apprentices try to kill him before", I'd bet against the apprentice every time.

Answer 1

There are two possible "answers":

1. Disney Canon: Not enough info for a definite answer, but Sidious was likely just Sidious.
2. Legends: A definite answer is still not fully available, but Sidious was almost certainly just himself.

Disney Canon

Not surprisingly, there is little information about this available at all in Canon. The closest comes from the this answer which references the twitter account of a high level employee who confirms that Plagueis is indeed dead (although the account in question is not canon).

Legends

Not surprisingly, far more details are available in Legends, although a definite answer is still not certain there.

A never-ending line of Sith can be ruled out by simply looking at Darth Sidious' master, Darth Plagueis. Darth Plagueis was trained by Darth Tenebrous. Darth Tenebrous did indeed have plans for "possessing" the body of his apprentice using some specially crafted "maxi-chlorians". When Darth Plagueis betrayed him, Darth Tenebrous took his opportunity to "infect" his apprentice's body. However, he immediately had a premonition of Plagueis' death at the hands of Darth Sidious, and promptly fled his body. His disembodied spirit remained trapped in the cave for (possibly) millions of years.

Therefore the idea of body hopping is indeed possible in Legends, and at least one Sith Lord attempted it. However, it's also clear that there isn't an unbroken line of possessions leading back to some ancient Sith Lord. Since there is no evidence that Darth Plagueis learned the body hopping trick himself, there is no reason to believe that Darth Sidious was anyone other than himself.

Related, this is all in general agreement with the answers in this very related question.

Answer 2

Sidious is simply Sidious, obviously. In the first 6 movies, obviously that's the intention. Even in The Rise of Skywalker, he's still the one in control even if the other Sith Lords' spirits literally live inside him:

So the answer is yes, Sidious is Sidious, even if the other Sith's spirits have been literally inside him since The Phantom Menace.