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A DNS entry for ‘domain_name’ already exists. You must remove this DNS entry from all servers in the DNS cluster to proceed.

While adding addon domain under cPanel, I was getting error as below –

A DNS entry for ‘domain_name’ already exists. You must remove this DNS entry from all servers in the DNS cluster to proceed.

1. Please make sure the domain which you are trying to add the addon domain should not present as addon/parked(aliases)/sub-domain on the same server.

2. If it is not present then check the DNS entry in DNS zone file. You will need root SSH access for it. SSH to server and check the below file –
/var/named

3. If there is entry of domain then you can remove/kill the DNS entry using this command – 
/scripts/killdns domain_name

Please give a try now, it should work! You can verify the DNS entry by firing the using this command – 
cat /var/cpanel/userdata/$user_name/main

4. If you are still facing the same issue then you will need to check the other servers with the same DNS cluster. There should not be entry of domain on the other server which you are trying to add.

If there is DNS entry then you will need to remove that entry. (Please make sure the domain which you are trying to add is present as addon domain/parked domain(aliases)/sub-domain).

Please take backup of DNS zone file before editing it (for safe-side). You can fire below command for it –
cp -rp /var/named/domain_name.db /var/named/domain_name.db_backup