When a participant returns for a new event, the Participant Center will automatically update all of their contacts. However, it will not copy over duplicates. Whenever the Participant Center copies over contacts, it filters out duplicate entries based on email addresses and not name. Your participants will want to review their contacts to make sure the email address did get added into their new Participant Center and will need to recreate any duplicate entries (based on email) they may have had in the previous year's Participant Center.