The double spend problem for domains is the double domain purchase problem. I claim example.com and you also claim it, who wins?
I'd like to see DNS ownership combined with the time range and an owner-unique key so you can go to the website you intended to visit rather than whoever happens to own the domain at the moment.
Whoever buys it first. With a centralised database, this is trivial.
Usernames are the same. Who gets @example.bsky.social ? Whoever registers it first. Problem solved.