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Do you have a reference for that? It seems hard for a federated system to enforce such a constraint. And what would be the use? that's what the @domain is for.

Mastodon have the same problem.

> https://social.tchncs.de/@voidnill > https://mastodon.cloud/@voidnill

are the same user (me) on two different instances.

What do you mean by "problem"? Email has the same "problem".

You can address them differently if you add the server like this @rocky1138@kwat.chat @rocky1138@gnusocial.no, at least on gnusocial

Mind elaborating what problems are you facing now?