Hmmm you mean first reincarnation since the historical arc should be the original, then after that should be the first reincarnation, then the modern day is second reincarnation. if Sunbae's grandfather is his own self in the first reincarnation, then that means he wont die in the second reincarnation
But so far he is not married in the first reincarnation, nor does he have biological kids. So how would he end up with a grandson that looks just like him? And if the grandfather is the first reincarnation and doesn't die, then how is he reborn into the modern incarnation then? Or is this what you were meaning to point out?
I dunno yet, we've only got two chapters out yet. But i remember Sunbae's mother saying that he looked just like grandpa and how grandpa really liked him (but tbh that confused me bc if it was actually his previous life then how did the two interact?) And it got my brain rolling. Im really excited to see how far this new mini story goes (☆▽☆)
This has to be their second reincarnation and the one we read is the third! Because its way too soon chronologically, the one we read is modern day and this one is during the war. Besides, the mother and everyone says that Sunbae looks like his Grandfather. Which fits in to the timeline almost perfectly (⊙…⊙ )