As you know, when add a new domain in your cPanel, cPanel use an internal subdomain to name this domain
ie :
The main domain of your cPanel account is : maindomain.com
You add domain : addondomain1.com
cPanel will create internally a subdomain like addondomain1.maindomain.com
What is issue, in some cases, pages of your site maindomain.com will be accessible (also index by google) through URL addondomain1.maindomain.com
To fix this, create a file nginx.conf in the folder of related website, and add the following by adapting to the concerned domain values :
if ($host = addondomain1.maindomain.com) {
rewrite ^/(.*) https://www.maindomain.com/$1 permanent;
}
if ($host = www.addondomain1.maindomain.com) {
rewrite ^/(.*) https://www.maindomain.com/$1 permanent;
}
After, go to Nginx manager in your cPanel, and reload nginx