If you see a notice saying "This email address is already registered. Please enter a new one." you will need to enter a different email address. Only one Expo ID can be registered per email address.
If you have already registered your email address, please check your inbox to see if you have received an email containing the Expo ID registration link. If you have received such an email, please click the link to complete your registration.

If you do not remember registering your Expo ID with the email address you entered, please contact us using the contact form.

https://faq.expo2025.or.jp/hc/en-gb/p/contact