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While Adding the new domain (.com domain ) as an addon please make sure that the "Subdomain/Ftp Username" which is getting created is not similar to that of your .co.uk domain.
Eg…. When you add harry.com as addon domain you will get harry as Subdomain/Ftp Username similarly when you add harry.co.uk you will get same harry as Subdomain/Ftp Username which is causing above error [Error from domain wrapper: Domain already exists ]
Solution : When you add other domain name just try to change Subdomain/Ftp Username entry to something different from harry like harryUK or harry1
Hope this helps you