You get this error sometimes, when you are hosted using the cPanel
I parked yourdomain.com which redirects automatically to yourdomain.org.
I deleted the domain from Parked Domains panel, and added it back via the Addon Domains panel.
Then the error “Already Exists”, even though it was not listed in the Addon Domains nor was it listed in the Parked Domains.
This is due to the domain name existing anywhere in the cPanel configuration and need to get removed / deleted.
Check whether zone exists or not
dig @server_ip yourdomain.com
If the zone file exists, it will show the A record of yourdomain.com.
If zone file exists, log into the server and make sure the domain doesn’t exist
If it does, you need to remove prior adding the Addon domain.
Remove the zone file from the server
Now you have removed DNS zone and help you add the addon or parked domain.
There are old traces of the domain on the server
Log into the server where the customer is seeing problems adding the domain and confirm that the domain does not exist on the server.
Check cPanel files
grep domain.com /var/cpanel/users/*
grep -R domain.com /var/cpanel/userdata/*
Edit file/s that are found and remove domain name the customer is trying to add.
You also may need to remove the all the files related to domain name.
Rebuild user domains database
Rebuild Apache to make sure all entires of bad domain removed.
service httpd restart
This should have all entries left behind from when this domain name was removed in the past no longer conflict, when you try to add the domain again.