(Perhaps you’re thinking of this https://www.lesswrong.com/posts/EKu66pFKDHFYPaZ6q/the-hero-with-a-thousand-chances)
There’s also a comic series with explicitly this premise, unfortunately this is a major plot point so revealing it will be a spoiler:
Transdimensional Brain-Chip
(Perhaps you’re thinking of this https://www.lesswrong.com/posts/EKu66pFKDHFYPaZ6q/the-hero-with-a-thousand-chances)
There’s also a comic series with explicitly this premise, unfortunately this is a major plot point so revealing it will be a spoiler:
Transdimensional Brain-Chip