On a cPanel server if you get the error: Error from park wrapper: domain.com is already configured while adding an add-on domain under cPanel. Make sure to remove the domain.com entries from:

/var/named/domain.com.db
/etc/httpd/conf/httpd.conf
/var/cpanel/users/username
/etc/userdomains
/etc/localdomains

Renaming or commenting will not help; you will have to remove the entries completely. Try now.

This worked for me.

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