There are three ways of accessing an add-on domain when you add it in cPanel.
Root domain name (ie. www.addondomain.com)
Subdomain (ie. www.addondomain.maindomain.com)
Subfolder (ie. www.maindomain.com/addondomain)
So to aviod duplicate content issues in terms of SEO better redirect these Subdomain and Subfolder to “www.maindomain.com” or “www.addondomain.com”?
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The best option is to use names for those that are hard to guess and never link to them so that only your selected domain name appears in anything that links to the content and therefore the search engines (and real people) never know what subdomain and folder name that are used with it.
The only time you should ever use the subdomain name is in accessing the stats for the domain and the only time you should ever use the folder name is in updating the content of the site.
So with the following as your three entries only the first ever appears anywhere that anyone but you can see it and you make sure that the other two are names that no one is likely to guess.
Root domain name (ie. www.addondomain.com)
Subdomain (ie. www.xyzabc.maindomain.com)
Subfolder (ie. www.maindomain.com/def)