Issue
When attempting to add a custom domain, the command returns an error that the domain is "currently in use by another app."
Resolution
Although you may currently own a domain, it may have been added to another app in the past and never removed by the previous owner. This can be quickly resolved by our Domain Release tool.
Using that tool, you can specify which domain you're trying to regain control of. It will then ask you to add a DNS "TXT" record on the apex/root domain (ex. mydomain.com). Once the TXT record is verified by our systems, the domain will be released and you can add the domain to your app with this command: heroku domains:add mydomain.com.
The Domain Release app is here: https://tools.heroku.support/domain-release and further documentation about adding custom domains can be found here: https://devcenter.heroku.com/articles/custom-domains.
If you still face any issues please create a ticket here - https://help.heroku.com/tickets/new?id=8.