How can I verify my custom domain using domains.google.com
if
it doesn't allow duplicate keys and firebase hosting requires a duplicated key entry?
And firebase requires 2 text entries with same host.
DNS records in Google Domains can contain multiple values. So while you can only have a single TXT record for a given name, it can contain all the values you need to validate the domain for Firebase Hosting.
The steps are:
Here's one of our domains, after step 1: