I have purchased 2 domain names & 1 hosting plan from Godaddy.
I have added 1st domain as primary, and second one as addon domain.
But I can't create Addon Domain for the 2nd domain.
It's said 'The domain "mydomain.com" already exists in the userdata '
See screenshot: http://www.hotelcambodiana.com.kh/beta/Error-addon-domain-2.png
Check all references to the domain. Remove any domain aliases, subdomains, redirects that reference that domain. Also be sure to delete any FTP accounts and folders under public_html associated with it. If any of those exist it will not let you create an addon domain. Hope this helps
Had the same problem at one time. They way it was explained to me was that something on a server needed to be removed. It was like a ghost domain that didn't properly get cleaned when I had the domain name hosted on an earlier hosting account. My phone tech had to get ahold of the 'server guys' he said, but they got it fixed.