Spoiler No. It's Xinghai. Remember, in the diary, Yang calls his brother a person who always wins. The title is Forever Winner, because the older brother won the prize. Poor Xingyang tried to give him a clue with the diary but ML is just too dumb to understand. People typically know their family better than anyone else. When Xingyang wrote that it's impossible for others to tell the difference, it should've been a clue but ML was too convinced of his own impressions despite not knowing them very well. So once you know the truth, small events like giving back the diary and the text messages on the phone ML had to pry away from "Xinghai" to give back to "Xingyang" has a different meaning. The words of their housekeeper, who does know the truth(since he's the only one who can tell them apart), also comes with a different meaning. Makes you want to reread, right? I like this story because it has A Rose for Emily vibes(my favorite short story).
I guessed something like this from the name too. I thought at the end ML will hear them talking and thus the truth will be revealed. Well i wish the story had explained more as to why and how the MC and the twin switched.