There's a big chance this has been brought up before, but I'll say it anyway.
The reason people already know Sora (like Neku) is because they have already met him. Sora was asleep for a year after the events of Chain of Memories, so he has probably already visited the "world enclosed in sleep". That just leaves the question of "why doesn't he remember" and "What happened while he was there?"
The reason people already know Sora (like Neku) is because they have already met him. Sora was asleep for a year after the events of Chain of Memories, so he has probably already visited the "world enclosed in sleep". That just leaves the question of "why doesn't he remember" and "What happened while he was there?"