@lashauk
I'm not sure I fully understood the last part of your comment
It's actually quite simple, they are actually two different people.
You have the original owner of the body,
and then you have the MC who is the new owner of the body.
Basically: powerful person dies. His soul wanders
Then takes over the body of some ordinary kid who also died. Decides to take over the kid's identity as well.
The soul and the body were separate people, and the "normal guy" was actually just the dude who previously owned the body.
But since MC is the only person who knows about this "reincarnation", everyone assumes it's the same person even though the soul is different.
Altho usually in these settings, MC also inherits the memories of the previous owner, so he "takes over" the new identity as well.
However, MC usually still has a past history with different characters, so usually those two identities eventually converge.
It's more of a gimmick nowadays so most series don't address this much anymore, nor explain it in detail since it assumes most readers already knows about the trope.
But you do occasionally still have series which explores how MC's don't adapt to the new identity and gets an identity crisis, as their new family and friends are essentially just strangers/ or conflicts created once others find out MC is a different person quite literally. It just doesn't happen often in power fantasy Xianxia's.