We have received a user sync error, "Emails value has already been taken", but it does not appear those emails have already been taken as the error suggests.
- Studio | People > Users; Classic Studio | Manage > Users
No user is found for an email address despite email being taken if it is not a user’s primary email.
There is currently no known resolution for this issue.
Workaround: Audience builder can be used to do a more advanced email search using the "Email" filter.
Please check the current status of this issue on the Known Issues Tracker. (Issue ID: CSUP-561)