We had the same issue. Workfront allows the same email across multiple instances, but Workfront Proof only allows the email to be in the system once across all instances. If you log a ticket, they can help tell you where the email address is active. For our internal users that this happened with, we had IT add an alias to their email address so they would still receive notifications. For external users, we have to send them a direct link to their proofs or their IT will need to add the alias as well (we haven't pursued the latter).
