I would like to know if Peter van Daan and Peter Wessel in "The Diary of Anne Frank" are the same person.

I hate to confuse you even more, but the character's real name was Peter van Pels. After hearing a radio speech by a Dutch politician who encouraged people to write down their wartime experiences, Anne decided to revise her diary for future publication. Perhaps it was then that she decided to change the van Pels family into the van Daans.

Peter Wessel is a different boy on whom Anne has a crush. She hopes that they will find each other again someday after the war. She also develops a crush on Peter van Daan, so it is natural that you'd be confused!