It's the similarities that get to him, probably, the sense that it is more than just similarities, more than just the identical voice, the identical face (which, even after millenia, Shen Wei would not forget, ingrained as they are into the very core of him); the feeling that it's more than just a case of Kunlun being born again.
Shen Wei just tries not to think too much about it, tries not to get his hopes up, tries, oh how he tries to keep these two versions of… probably-unlikely-maybe the same man, separate in his mind.
After all, Kunlun was Kunlun and ten thousand years ago, and Zhao Yunlan is Zhao Yunlan and here and now, and, no matter how much his heart might be screaming that they're one and the same (because Kulun had promised that they’d one day meet again, he’d promised) despite how impossible that might be, Shen Wei knows this.
(Neither man would appreciate it, he also knows, Zhao Yunlan least of all, being compared to someone else, someone who doesn't even exist in their respective lifetimes.)
The damn lollipops don’t help matters, either.
Because he knows they’re a human invention, and a relatively recent one at that, not something humanity had such a long, long ago, but he remembers the sweet taste of them from an era of unending wars and carefully rationed food. It was the first sweet thing he’d tasted since before his parents died and Shen Wei remembers being fascinated ( well, by the lollipop and… yes. Well ), and he remembers not finding any in Dixing and being disappointed.
(He still has that one wrapper, picked up on a whim and stashed away before anyone, Kunlun especially, could see him do it. He’d felt like a child when he’d taken it, or like a young maiden in love, but soon enough it became the only tangible thing he’d still had of the man who came into his life like a whirlwind and left just as devastatingly quickly.)
The first time he sees Zhao Yunlan sucking on one, ten thousand years and a few days later, Shen Wei wants nothing more than to take it out of the man’s mouth and suck on it, if only to see if it would taste the same.
It takes his brother escaping his imprisonment to finally find out the whole truth of it.
That the man he fell in love with in his youth, there and gone in the blink of an eye, is indeed the same man he fiercely loves now, that it was never a case of rebirth or reincarnation or just the universe messing with him, but that it is, in fact, the same man, ten thousand years apart.
Zhao Yunlan is Kunlun is Zhao Yunlan.
And, yes, he also finds out, the lollipops do taste the same.
(Like green apples and Zhao Yunlan).