Why not? A 20 digit long string of numbers is a perfectly valid visibleID. It corresponds with the visibleID of 1e+58 other accountIDs
Are we all talking past each other here? Are we all even talking about the same thing???
When someone else enters a passphrase that gives already used account they'll see a big red message saying that this account can't be used.
That might be a problem for offline or paper wallet creation. Something I would like to see in future. Since when offline it can't be known if an account already exists.