What prevents other users from using "Add a domain" that they don't own?

I expect you have some tool that prevents this from occurring, but what prevents another user from duplicating your domain name in their own account, and creating a new server there? The reason I’m wondering is because without that tool, if your own website ever goes down, wouldn’t that other server start serving their own content? But then again, this doesn’t happen, so what prevents this? And if you delete the domain name from your own account, but leave the dns pointed at the host in your registrar, wouldn’t you be enabling someone else to use your domain? Or if you buy a domain, point the dns to a host like DO, and someone goes to “Add a domain” before you get around to it. I’m not facing any issues like this personally, I’m just wondering how all that works.