Originally, this was not the case. When ownership verification was added to custom domain publishing, in 2012, every use of the Publishing wizard - even for domains previously purchased using "Buy a domain" - would require a new "CNAME".
Since many registrars limit the number of address entries / domain, the new "CNAME" meant half the maximum number of hosts / domain. Of course, most blog owners patience ran out long before the magical maximum - but there was always that spectre hanging over every custom domain.
Recently, the domain ownership verification requirements changed.
Now, instead of requiring ownership verification for each host, only publishing to the domain root generates the apocryphal "Error 12" and variants. Some blog owners, publishing to non root hosts, or trying to publish with spurious addresses, find their domains published without even the hint of verification.
Successful domain publishing requires you to add righteous base DNS addresses, and get the domain working.
- If you add righteous base DNS addresses, and the domain works without ownership verification, ownership verification is not required.
- If you add righteous base DNS addresses, and ownership verification is required, instructions for adding the second "CNAME" will be provided, when you publish to the domain.
- If you add spurious base DNS addresses, the domain is not going to work - even with ownership verified.
If you get the base addresses right, you may be given instructions for the second "CNAME". If so, you return to the zone editor, add the second "CNAME" using the instructions, and repeat the Publishing wizard. If the Publishing wizard indicates success
mycustomdomainblog.blogspot.com redirects to www.mydomain.comyou are almost done.
Use a text proxy and verify that the domain is now operational. If the text proxy shows a problem, check your base DNS addresses - and correct if necessary. Allow for proper DNS latency, and check domain operation again.
If any doubt or confusion, start a new topic in Blogger Help Forum: Get Help with an Issue, and request assistance. Let us help you, before you try using a spuriously setup domain, and maybe avoid the old corrupt domain.
Key already exists for www.mydomain.comWhy subject yourself to needless pain and suffering?
Just don't worry, needlessly, if you don't get the well known "Error 12" when you hit "Save", and see
mycustomdomainblog.blogspot.com redirects to www.mydomain.comVerify operation of the domain, and start getting the domain indexed by the search engines.