Blogger / Google personnel provide Help instructions which are not always updated - sometimes they simply write a new Help instructions document, leaving the old Help instructions in place.
The policy of leaving old Help instructions in place - and sometimes conflicting with new displays and procedures - occasionally causes confusion. One scenario where this causes a problem is in publishing or re publishing a blog to a custom domain.
I've seen instructions that when you buy a domain from Blogger / Google, using "Buy a domain", you won't need to create a CNAME record.
One of the advantages of using "Buy a domain" is the ease of setup. If you use "Buy a domain", you don't have to bother with any of the details. This is generally - but not always - true.
Sometimes, even using the "Buy a domain" wizard can later present a problem.
One of the known problems with "Buy a domain" results in a partially setup domain - and with a partially setup domain, you may have to re publish the blog to the domain, to get the new domain to work completely.
To re publish the blog, you have to verify your ownership of the domain - even if you previously used "Buy a domain". And the additional step of adding a second "CNAME" overrides the old Blogger Help instruction
If you bought your domain name from Blogger, you won't need to create a CNAME record.When you are instructed to re publish the blog using "Advanced settings", that 's what you must do. And, when "Advanced settings" contains instruction to add a second "CNAME", then please - add a second "CNAME".