No future knowledge, no cheats, no isekai.
Random person from Re:Zero goes through their day normally until...
Subaru dies.
Everything returns as they were at Subaru's save point.
Except the main character of the fanfic.
Yes, he is not affected by it. It was dark outside and in the next second, they realized the sun is back up with his friends are gone.
Weirded out, he goes to his house to discover... himself.
And that's mostly the gist.
Random person literally got copied and Return By Death does not sent him back where he was. Once he dies, he dies as Subaru cannot bring him back.
It's an interesting concept from my view, but...
There are a few problems concering how his body would work. If he would age, gain muscles and etc. Frankly I do not have answers to this questions, the most I can say is that since he's a copy, Return By Death doesn't get rid of his memory and current location but does restore his body back to how it was yet if he dies, Return By Death simply won't realize a copy of another person existed to even revive him. Yes, it's peak bullshit.
(Can you tell I am in my Re:Zero era once again?)