[bjt2n3904]

Ugh. This just further poisons me against unicode being a good thing. ASCII or bust.

"ABCD" and "𝓐𝓑𝓒𝓓" are the same thing, but they also aren't. Am I supposed to normalize everything on username creation to prevent people from making duplicates?

[function_seven]

You either have to do that, or restrict usernames to ASCII subset only (and face the wrath of non-Latin alphabet users), or restrict to a whitelist of ranges that only represent alpha characters across all languages (i.e. not math symbols, nor box-drawing, nor emoji, etc.)

i8n is complicated because the diversity of language is itself complicated. I feel your pain, though, don't get me wrong.

[codebje]

Not normalise, just set the rules you want to apply. Internationalised domain names suffer this issue, so cribbing their fix probably would work for you - pick a script, restrict allowable characters to it.

You probably never want to allow unrestricted use of any character set for a username, even ASCII - otherwise I could take the username 'bjt2n3904 '.