Each email address can only be used once per HBS environment (QA and PROD). This is related to the Liferay user management that does not allow multiple uses of the same email address in one HBS environment. When this error occurs, another user account with the same email address already exists in the HBS environment. This can happen when the partner is already linked into another partner role (e.g. CPO/EMP).
We recommend checking with your email provider if they support the use of tags, e.g. firstname.lastname@example.org instead of email@example.com.