What's the point for author to use visible ID?
Do u like riddles?
Alias System allows to create memorable addresses for payments, like "johnsmith". In Nxt everything is made on purpose...
I'm not saying the Alias System, but the 20 digits visible ID truncated from the full public key.
I think it shouldn't act like what you say as prevent user from generating conflicted id when the first 20 digital happened to be the same.
But give the user the longer id to distinguish between existing and and new created one.
For example, if there was an existing id 11111111111111111111 with full key 11111111111111111111xxxxxxxx
then you generated a new id with full key 11111111111111111111yyyyyy which conflict in the first 20 digits
the system should give you the id 11111111111111111111y instead of rejecting you from creating new id.
In short, this mechanism should be like an embedded first-bit service that supported in blockchain.info.