I’m working a lot lately in Firebase and when trying to setup Firebase Hosting for one of my domains, Firebase showed me the following dialog:
Hopping over to Google Domains, I created the first A record, but when I added the second record, Google Domains reported “Record already in use”.
I poked around on the Internet for a while and didn’t find the answer in the Google Domains documentation or through a Google Search. I used the Google Domains contact form and was quickly on a chat where a support technician showed me the simple solution.
You can’t add two A records for the same domain, but you don’t have to - you can add multiple IP Addresses for a single record as shown in the following figure:
Put in the first address, click the highlighted + button to open a panel that allows you to add the second address. Easier than expected, but hard for me to notice that button.