Like you get transported into an alternate universe where everything is almost same, but with tiny differences. The world looks normal to everyone and you’re the only one that have memories of the differences.
Do you think you would trust your memories or would you think you lost your mind?
Neither explicitly. I would seek more information. I would expect to be caught up in a The Leftovers type situation (2% of the world’s population inexplicably vanishes in an instant). That is to say, I would not expect to be the only one affected.
Either way, I would also seek evidence this person ever existed. If I’m truly the only one who remembers them, maybe the problem is with me. Logically speaking, I mean. There is a school of thought called “Last Thursdayism” that says that everything before last Thursday (or some abstract point in the near past) was invented by our brains. The break between short-term and long-term memory. The film Dark City plays upon this concept. (“Does anyone know how to get to Shell Beach?”)
Past a certain point, I’d be very careful who I speak to about something that seemingly I am the only person who believes differently from the rest of society.