CloudFlare: domain already exists
Zone 'mydomain.com' already exists
can be returned by CloudFlare when you are trying to add one of your Servd domains to your CloudFlare account via the Servd dashboard.
This error is returned when a different CloudFlare account has already added the domain and has verified it by setting the domain's nameservers to point towards CloudFlare. However, this doesn't mean that the account is still being used and might be left over from a legacy setup.
CloudFlare's block on adding these domains only applies to their API, so you can still add the domain to your CloudFlare account manually.
Visit cloudflare.com and log in using your CloudFlare account's credentials
If you don't yet have any sites you'll be dropped into the new site flow. If you already have sites you'll need to click the '+ Add Site' button at the top of the page.
Proceed through the site creation flow, following the instructions until the site has been successfully added.
Return to the Servd Dashboard and attempt to add the domain to your CloudFlare account again. It should now succeed, linking your Servd project to the domain you just added to CloudFlare.
If you'd like to sync your DNS records on CloudFlare, open the DNS section within the CloudFlare panel and click 'Update DNS For Servd'.