Now you are just making up implausible scenarios that don’t help.

A chat app or any app with a PM feature either has a globally unique user name feature or has an internal identifier for the user so the user-chosen name doesn’t have to be unique. In the former case, any user will be able to see two seemingly identical user names in their chat list, but one with no chat history. In the latter case, well humans are known to have duplicate names too, so Cyrillic characters don’t even come into the picture.