When you see the error "The phone number is already registered"
There is possibility that you have accidentally created an account and registered the phone number.
Please make sure to enter the correct email address and try logging in again.
Also, Coincheck allows only one account per user. If you have created multiple accounts, please use the one which you have registered the phone number with.
If you think you have not created any account and registered your phone number, please contact us from the inquiry form.