cPanel: The domain already exists in the userdata.

Where to check:
whmapi1 domainuserdata domain=somedomain.com
grep -Rl somedomain.com /var/cpanel/users/
grep -Rl somedomain.com /var/cpanel/userdata/
grep -Rl www.somedomain.com /etc/apache2/
grep -Rl somedomain.com /etc/*domains

After that:
/scripts/updateuserdomains
/scripts/updateuserdatacache
/scripts/rebuildhttpdconf
/scripts/restartsrv_httpd