I wonder if your work will cover the case when someone displays a different username than theirs in their signature. Example: [[user:Foo|Bar]]
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This shouldn't be allowed since it is very confusing for everyone. I've already seen people being wrongly pinged, or seeing Foo complaining because someone pinged user:Bar instead of them.
The counterargument "my username is already taken, I want to display my real identity" is a false one. No one complains because they get a phone number they don't like. When one wants to change their username, they can if the account is not taken, or if the account is not in use any more. It is done often. Already used and active usernames simply follow the "first arrived first served rule" and it is clearly not much of a big deal. It happen to me back in the days: my current username was my backup nickname on the Internet. I had the opportunity to built a new identity which was a great experience.