Follow the steps below to rectify this issue.
1. Under the contacts module, click on All Contacts view. Try searching if there is any contact with the same email address by mentioning it in the Primary email, and see if you are able to find any contact. If you do find any contact, remove the email address from the contact info and try adding the support rep with the mentioned email.
2. If the above report doesn't return any value, go to reports - > New query report, and execute the below query.
select * from aaausercontactinfo where contactinfo_id in(select contactinfo_id from aaacontactinfo where emailid like '%emailaddress%')
- > Replace the email address with the user's email address.
3. You will be seeing the userid for the contact. Open any contact and replace the userid in the URL; check if you are able to see the contact details.
4. You can remove the email address from the contact details and save it. You should then be able to create a contact with the mentioned email address.