In general, this is about a user has multiple accounts associated to the same email address. (e.g. a user could register at service market place and another SAP site/app in which user registration are centrally managed by SAP ID service.) When user tries to log in cloud with his/her email address, cloud cannot tell which account number is the primary one.
Product team will need to remove the unwanted association then user can register successfully. In future this should happen much less since SAP ID has improved the registration process.